Customary international law is a central part of the international legal order and, as established in the earlier Foundation chapters, is one of the recognised sources of international law. This chapter now moves beyond that introductory identification and examines the customary-law process in depth. The question is not simply whether States behave in a particular way. The legal task is to determine whether the evidence establishes a rule of international law through the requirements applicable to customary international law.
The student should therefore approach this subject as a problem of legal identification. When a party asserts that a customary rule exists, the court must first identify precisely what rule is alleged. A broad statement such as “international law requires States to act fairly” is not sufficient. The court needs to know the particular obligation, prohibition, permission or legal standard said to bind the relevant States. The more precisely the alleged rule is formulated, the more accurately the court can identify the evidence needed to establish it.
This approach builds directly on the completed Foundation course. F1 established the general architecture of international law and introduced the recognised sources. F2 showed how international law developed through changing patterns of relations between States and institutions, while distinguishing historical development from proof of present law. F3 examined customary international law within the wider law of sources. F4 established the substantive framework of treaty law. F5 therefore has a more specific task: to teach how a customary rule is identified, evaluated and applied when its existence, content or scope is disputed.
Customary international law is commonly identified through two essential elements: relevant State practice and acceptance of that practice as law, known as opinio juris. These elements are related but perform different functions. State practice concerns conduct and other authoritative acts of States. Opinio juris concerns the legal character attributed to that conduct. A court must examine both rather than treating one as a substitute for the other.
State practice may take many forms. It can appear in diplomatic correspondence, official statements, legislation, administrative conduct, military manuals and instructions, national judicial decisions, treaty practice, conduct concerning international organisations, and other authoritative governmental acts. The importance of a particular item depends upon its source, context, consistency, specificity and relationship to the legal question before the court. A single statement may therefore have a different evidential value from a consistent pattern of official conduct extending across many States.
Opinio juris requires a separate inquiry. States may behave in a similar manner for many reasons. They may do so because the conduct is convenient, politically desirable, economically useful, customary in an ordinary social sense, technically necessary or considered good policy. Such conduct does not automatically demonstrate a legal obligation. The court must ask whether the State's conduct is accompanied by the understanding that the relevant conduct is legally required or permitted under international law.
This distinction prevents an important error. Repetition alone does not transform conduct into law. At the same time, the absence of a treaty does not mean that no legal rule can exist. Customary international law provides a distinct route through which international legal obligations may arise and operate. The legal question is always whether the evidence satisfies the requirements for the particular customary rule alleged.
The two elements should not be treated as completely isolated tests. Their relationship is central to the inquiry. Evidence of State practice may help establish what States actually do, while statements, protests, legal submissions and other expressions of legal position may help establish why States consider that conduct legally significant. Some materials may provide evidence relevant to both elements. The court must nevertheless identify what proposition each piece of evidence actually supports.
This is important because identical conduct can have different legal explanations. Suppose many States follow the same administrative procedure. That may demonstrate considerable uniformity, but the reason for the uniformity remains relevant. The States may have adopted the procedure because it is efficient, because domestic law requires it, because it is diplomatically convenient, or because they believe international law requires it. Those explanations have different legal significance. The court must therefore examine the surrounding material rather than treating observed conduct as self-explanatory.
The reverse problem can also arise. States may repeatedly assert that a particular principle is required by international law while their actual conduct is inconsistent, limited or selective. Such statements may be important evidence of opinio juris, but they cannot simply replace the inquiry into practice. A State may acknowledge a rule while claiming an exception, disputing the facts of a particular incident, or arguing that another rule justifies its conduct. The court must identify which explanation the evidence supports rather than assuming that conduct or words alone provide the complete answer.
The distinction between legal obligation and political preference is therefore fundamental. International relations contain many statements in which States support desirable standards without asserting that those standards are already binding law. A government may call for stronger environmental protection, greater humanitarian safeguards, improved diplomatic practice or a preferred standard of international cooperation. Such a statement may be politically significant and may contribute to the development of a legal norm, but it does not by itself establish a present customary obligation. The court must determine what legal proposition the State has actually asserted and whether the wider evidence supports it.
Habit presents a similar difficulty. Conduct may become habitual because institutions have used the same process for many years. Officials may follow established administrative routines without considering whether international law requires them to do so. Long duration can therefore be relevant evidence, but duration alone does not supply opinio juris. A practice can be old without being customary, just as a comparatively recent practice may become legally significant if the evidence demonstrates the necessary elements.
Convenience must likewise be distinguished from legal obligation. States frequently cooperate because cooperation produces practical benefits. They may follow reciprocal procedures because doing otherwise would create diplomatic difficulties or economic costs. Such conduct can be widespread and stable while remaining essentially pragmatic. The court should therefore avoid reasoning backwards from regularity to legality. The proper question is not simply whether States usually behave in a particular way, but whether the evidence demonstrates that the relevant conduct is treated as legally required or permitted under international law.
The practical significance of this distinction is that customary international law cannot be identified merely by counting examples. Evidence must be connected to the legal proposition being proved. Ten States acting alike for purely practical reasons do not necessarily establish a customary rule. Conversely, a smaller body of particularly clear and legally reasoned practice may be highly significant, especially where the subject matter concerns States with direct involvement. The court must assess the quality and legal significance of the evidence rather than applying a mechanical numerical formula.
Article 38 of the Statute of the International Court of Justice provides the familiar formulation of international custom as evidence of a general practice accepted as law. F3 has already examined Article 38 within the wider law of sources. In F5, its importance is methodological: the court must move from the recognition of custom as a source to the concrete task of establishing the alleged rule. Article 38 does not itself prove that a particular rule exists. The evidence must establish the rule through the applicable customary-law analysis.
Generality and consistency are consequently important considerations. A general customary rule normally requires practice that is sufficiently widespread and representative in relation to the subject matter. The number of States involved is relevant, but numerical counting alone is not the legal test. The court must consider which States are affected by the activity, whether the practice is representative, whether the conduct is sufficiently consistent and whether States treat the practice as legally significant.
Consistency also requires careful analysis. Occasional departures do not necessarily disprove a customary rule. A State may breach a rule while still accepting that the rule exists. It may describe its conduct as an exception, justify it on another legal ground, or dispute the facts without denying the underlying rule. Conversely, repeated and legally reasoned rejection of the alleged rule may provide substantial contrary evidence. The court must therefore examine what States say about departures as well as the departures themselves.
Customary international law can develop gradually. A practice may begin as a limited or contested pattern of conduct and later acquire wider legal significance. Conversely, a developing practice may never satisfy the requirements for a customary rule. The court must distinguish an emerging practice from an established rule and must not convert political expectations or aspirations into binding law without sufficient evidence.
History can assist with this inquiry, but it has a limited role. Earlier Foundation teaching established that historical practice can explain how international rules developed while also showing why historical conduct does not automatically constitute contemporary law. In F5, historical evidence may establish the chronology of a practice, identify earlier State positions or explain how a legal claim developed. The decisive question remains whether the evidence establishes the alleged customary rule under contemporary international law.
The relationship between customary international law and treaties is especially important. F4 has already taught how treaty obligations arise and operate, including the importance of consent and the legal operation of treaty commitments. F5 therefore approaches treaties from a different direction. The question is whether a rule contained in or reflected by a treaty also exists independently as customary international law.
A treaty provision may reflect an existing customary rule. It may also contribute to the development or crystallisation of a customary rule. Alternatively, it may establish obligations solely through the treaty relationship for States bound by that treaty. These possibilities must not be confused. A provision does not become customary international law simply because it appears in an important multilateral convention.
Consider a hypothetical multilateral convention containing a rule governing a particular form of State conduct. A court faced with a dispute involving a State that is not party to the convention cannot simply reason that the convention's provision binds that State because the convention is widely supported. The court must investigate whether the same rule exists independently as customary international law. It may examine the conduct of States, statements made about the legal character of that conduct, reactions to violations, subsequent practice and other relevant evidence. The convention may be highly important evidence, but the court still has to establish the customary rule independently.
The reverse is equally important. A State's non-participation in a treaty does not automatically mean that the State is free from a rule that independently exists as customary international law. If the evidence establishes an applicable customary rule, its legal basis is different from the State's treaty consent. The court must therefore identify whether the obligation is treaty-based, customary, or supported by both legal bases.
This distinction between legal authority and evidence is fundamental throughout F5. A treaty may provide evidence relevant to custom without itself being the source of the customary obligation. A judicial decision may identify and explain custom without itself becoming the source of a general rule. A resolution may demonstrate that States have expressed a legal position without automatically establishing that the proposition has become customary international law. Each item must be analysed according to the proposition for which it is being relied upon.
International organisations and their resolutions can therefore contribute to the evidential picture. An organisation may record State positions, provide a forum for repeated governmental statements, or produce resolutions concerning international legal principles. But an institutional resolution should not simply be equated with universal State practice and opinio juris. The court must consider how it was adopted, what States said, whether the wording was intended as a legal position, and what subsequent State conduct demonstrates.
International judicial decisions have a related but distinct role. Decisions of international courts can be highly persuasive when identifying customary rules because courts examine State practice, opinio juris and competing legal arguments. Their reasoning can clarify evidential requirements and the scope of a rule. But the court must distinguish identification of an existing customary rule from creation of a new rule by judicial assertion. The legal basis for the conclusion must remain visible.
Scholarly writings have a subsidiary function. They may assist by organising evidence, explaining competing interpretations and identifying patterns in State practice and judicial decisions. They do not become binding law merely because a respected scholar has stated a proposition. Their usefulness depends on the quality of the analysis and the evidence they identify. The court must ultimately ground its conclusion in recognised legal sources and relevant evidence.
The same evidence may therefore perform different functions in different parts of the inquiry. A national statute may demonstrate governmental conduct and may contain language revealing a legal position. A diplomatic protest may reveal a State's understanding of an alleged obligation. A national judicial decision may show how a State's legal system approaches an international rule. A treaty may provide evidence relevant to the development or recognition of a customary rule. None should be assigned automatic legal weight without examining its context and the proposition it is being used to establish.
This is also why contrary State practice cannot simply be removed from the analysis because it complicates the preferred conclusion. A proper inquiry must consider evidence pointing in both directions. If States have expressly rejected the alleged rule, the court must examine those objections. If States have departed from an alleged rule while continuing to describe it as legally binding, the court must consider whether those departures are breaches, exceptions or evidence against the rule. The conclusion should follow from the assessed evidence rather than from selective examples.
Specially affected States may have particular evidential importance where certain States have unusually extensive and direct involvement in the activity from which the alleged rule arises. This does not mean that only those States matter, nor does it establish an automatic numerical rule. It requires the court to consider the representativeness and quality of practice in light of the subject matter. The later sections will examine this issue in greater depth.
The persistent-objection doctrine presents another specialised question. Where a State consistently and clearly objects to a customary rule while that rule is developing, the court may need to consider whether the requirements for a persistent objection have been satisfied and what consequences follow. The doctrine cannot be reduced to the proposition that any disagreement defeats custom. Timing, clarity, consistency and the surrounding legal circumstances matter. Its detailed operation belongs to the later examination of formation and evidence.
Custom may also have a geographical scope narrower than the international community as a whole. A claimed rule may be general, regional or local. The court must identify the scope alleged before assessing whether the evidence is sufficient. Evidence establishing a regional practice cannot automatically be treated as proof of a universal customary rule. The legal question must always be matched to the geographical and substantive scope of the evidence.
The court's practical method can therefore be expressed as a disciplined sequence. First, identify the precise customary rule alleged. Second, identify the relevant States and the geographical or personal scope claimed. Third, collect the relevant evidence of State practice. Fourth, assess its quality, consistency, generality and representativeness. Fifth, identify evidence concerning opinio juris. Sixth, consider contrary practice, objections, silence and competing explanations. Seventh, examine treaties, institutional materials, judicial decisions and scholarship according to their proper evidential or subsidiary functions. Eighth, determine whether the evidence establishes the rule and its scope. Finally, apply the established rule to the proven facts.
A sound judicial finding should make that evidential path visible. If the court finds that a customary rule exists, it should identify the relevant practice, explain its generality and consistency, identify the evidence of opinio juris and address significant contrary material. If the evidence is insufficient, the court should explain which element or evidential requirement has not been established. This makes the reasoning reviewable and protects the distinction between identifying law and creating law.
The purpose of this introduction is not to make the student memorise a formula without understanding it. It is to establish the reasoning discipline that the remainder of F5 will develop. Section 2 will examine the definition and legal structure of customary international law in greater depth. Section 3 will examine State practice. Section 4 will examine opinio juris. Section 5 will examine formation and development, including the relevant doctrines concerning time, generality, specially affected States and persistent objection. Section 6 will bring the different forms of evidence together and examine how competing evidence should be evaluated. Sections 7 and 8 will then prepare and revise the complete framework for examination and practical application.
The central lesson is that customary international law is neither mere habit nor unwritten policy. It is a recognised form of international law whose existence must be demonstrated through the applicable legal requirements. The court must distinguish conduct from the legal belief accompanying it, evidence from legal authority, historical development from contemporary law, treaty obligations from independent customary obligations, and political preference from legal obligation.
The student should therefore leave this introduction understanding why customary international law requires specialist analysis. The earlier Foundation chapters established the legal landscape. F5 now takes the next step: it teaches how to prove, challenge, distinguish, develop and apply a customary rule without confusing repeated behaviour with law or judicial assertion with independent legal authority. The later sections will supply the detailed rules and evidential analysis needed to perform that task rigorously.
A further distinction is necessary between proving that a customary rule exists and proving precisely what that rule requires. Evidence may support the existence of a general legal principle while leaving disagreement about its exact content, exceptions or scope. A court should therefore formulate its conclusion with the same precision as the evidence permits. It should not take evidence supporting a broad customary principle and automatically convert it into a detailed obligation that the evidence has not established. In a real dispute, the difference can be decisive: the question may not be whether any customary rule exists, but whether the particular obligation asserted by one party is actually part of that rule and applies to the facts before the court. This requirement of precision helps prevent customary-law reasoning from expanding beyond the evidence and provides a sound basis for the detailed analysis developed in the later sections.
A useful illustration of this method is the International Court of Justice's reasoning in the North Sea Continental Shelf cases. The dispute concerned the continental shelf boundaries between Germany, Denmark and the Netherlands and required the Court to consider whether the equidistance principle contained in the 1958 Continental Shelf Convention had also become a rule of customary international law binding independently of that Convention. The legal question was therefore not simply whether the treaty provision was important or whether some States had followed an equidistance approach. The Court had to examine whether the evidence demonstrated the conditions necessary for an independent customary rule.
The case is important because it demonstrates the difference between a treaty rule and a customary rule while also showing how treaty material may become relevant evidence in a customary-law inquiry. A court must examine the practice of States, the circumstances in which they acted and the extent to which they accepted the relevant conduct as legally required. It cannot assume that widespread conduct automatically establishes custom, nor can it assume that inclusion of a principle in a treaty automatically binds States outside that treaty. The student should therefore ask not merely how many States followed a particular approach, but what the evidence demonstrates about the legal rule alleged and the legal character attributed to the practice.
The same discipline applies whenever a party relies on a convention, resolution, national legislation or judicial decision. Such material may be highly relevant without being independently sufficient. The court must identify the proposition for which the material is relied upon and then determine what additional evidence is required. This distinction between relevance and sufficiency is essential to reliable customary-law reasoning and will be developed throughout F5.
The North Sea cases also demonstrate why State practice cannot be treated as a simple numerical exercise. Practice must be connected to the particular legal proposition being asserted, and the court must consider why States acted as they did. A State may follow a particular course because of domestic law, practical convenience or reciprocal arrangements rather than because it believes international law requires that conduct. Conversely, a State may object to a practice as a matter of legal principle even where its own conduct is affected by practical circumstances. The court must therefore examine conduct together with the legal explanations surrounding it.
This approach illustrates a broader rule of judicial discipline. Evidence must be evaluated for the proposition it actually supports. A convention may provide evidence relevant to the development or recognition of custom without independently binding a non-party State. A resolution may record an important collective legal position without automatically proving universal custom. A national judgment may show the legal position within one State without establishing a general international rule. An international judgment may provide powerful reasoning and identify relevant evidence while the court must still identify the independent legal basis of the rule. The student should therefore ask two questions whenever authority is relied upon: what does this material establish, and what does it merely provide evidence of?
PUBLIC LEGAL REFERENCE
Customary International Law
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Learning Objectives
By completing this chapter, the learner should be able to work through the following objectives.
01
Define customary international law and explain its place among the sources of international law.
02
Explain the distinction between State practice and opinio juris.
03
Identify the principal forms of evidence used to establish State practice.
04
Explain how diplomatic statements and protests may demonstrate opinio juris.
05
Assess the relevance of national legislation and national judicial decisions to customary international law.
06
Explain the importance of generality, consistency and representativeness when assessing State practice.
07
Explain how customary international law may develop over time.
08
Distinguish an existing customary rule from an emerging or developing practice.
09
Explain the relationship between customary international law and treaty obligations.
10
Explain how treaties may codify, crystallise or contribute to the development of customary rules.
11
Explain the relevance of specially affected States.
12
Explain the persistent-objection doctrine.
13
Identify the evidential role of international organisations and their resolutions.
14
Explain the role of international judicial decisions in identifying customary international law.
15
Explain the subsidiary role of scholarly writings.
16
Apply an evidence-based method to determine whether a customary rule exists.
17
Identify and evaluate contrary State practice and objections.
18
Distinguish legal obligation from political preference, habit or convenience.
19
Construct a reasoned judicial finding concerning an alleged customary international rule.
20
Apply the customary-law framework to a practical international legal dispute.
SUBSTANTIVE TEACHING
Study Chapter
Open each section when you are ready to study it. Your legal text is preserved in full; the expandable layout simply makes the material easier to navigate.
Customary international law is a recognised form of international law established through State practice accompanied by opinio juris. It is distinct from treaty law because its legal existence does not depend upon a particular written treaty binding the States concerned. Its identification requires the Court to examine evidence of what States do and, critically, whether that conduct is accepted as legally required or permitted.
The definition must be understood as a legal test rather than a description of frequent behaviour. States regularly behave in similar ways because of diplomacy, economics, strategic interests, administrative convenience, reciprocity, technical necessity or ordinary international courtesy. None of these reasons, by itself, establishes a customary legal obligation. The Court must determine whether the conduct has the legal character required for custom.
The two constituent elements are State practice and opinio juris. State practice concerns the relevant conduct and authoritative expressions of States. Opinio juris concerns the belief or understanding that the relevant conduct is carried out because international law requires or permits it. They are analytically distinct but operate together. A conclusion concerning custom should therefore identify the evidence supporting each element and explain how the evidence combines to establish the alleged rule.
State practice is not confined to physical acts. Relevant material may include diplomatic correspondence, official statements, national legislation, executive measures, military manuals and instructions, national judicial decisions, treaty practice, conduct during negotiations, positions expressed before international organisations and actual conduct in international relations. The Court must assess each item according to its reliability, authority, context and connection with the legal proposition under examination.
The legal significance of practice depends upon context. An act undertaken during an exceptional crisis may have little value as evidence of a general rule if the State itself treated the circumstances as unusual. Conversely, a consistent pattern of official conduct accompanied by clear legal explanations may be highly significant. The Court should therefore avoid both extremes: it should neither dismiss conduct because it is not perfectly uniform nor treat every repeated act as proof of law.
Generality and representativeness are important because customary international law is not created simply by the practice of one or two States acting independently. The Court must consider the relevant international community for the rule being claimed and determine whether the evidence is sufficiently widespread and representative. This does not require every State to have acted identically. The assessment is qualitative as well as quantitative and must take account of the States particularly concerned with the subject matter.
Consistency must be assessed realistically. Occasional departures from an established rule do not necessarily destroy the rule. A State may breach an obligation while continuing to recognise that the obligation exists, or may claim that its conduct falls within an exception. Such conduct must be distinguished from a State expressly rejecting the alleged rule. The Court should examine the explanation accompanying the departure and the reactions of other States before deciding what evidential significance it has.
The distinction between practice and opinio juris becomes especially important when the same conduct can be explained in several ways. If States repeatedly follow a particular procedure because it is convenient, that practice may demonstrate regular behaviour but not necessarily legal acceptance. If States follow the procedure while expressly stating that international law requires it, the same conduct has additional evidential significance. The Court must therefore investigate the legal reasoning surrounding conduct rather than infer opinio juris merely from repetition.
Official statements can be particularly valuable when they clearly identify a State's legal position. Government statements, diplomatic protests, legal submissions and authorised declarations may reveal whether a State considers conduct required, permitted or prohibited by international law. Their weight depends upon the authority of the speaker, the circumstances in which the statement was made, its precision and its consistency with the State's other conduct.
Silence requires particular caution. Failure to object does not automatically demonstrate acceptance of a rule as law. A State may have been unaware of the conduct, may not have had an opportunity to respond, or may have considered a response unnecessary. Silence can acquire evidential significance only where the circumstances reasonably support an inference that the State was aware of the relevant practice and had a meaningful opportunity to react.
Customary international law may be general, regional or local. The Court must identify the geographical scope of the alleged rule before evaluating the evidence. A rule claimed to apply universally requires evidence directed to general international practice, whereas a regional or local rule must be examined principally through the practice and legal positions of the States concerned. Evidence of a regional practice cannot simply be converted into proof of a universal rule.
Custom also has a temporal dimension. A customary rule existing at one point in time cannot automatically be projected backwards or forwards without evidence. When a dispute concerns whether a rule existed on a particular date, the Court must examine the relevant period and avoid treating later developments as conclusive proof of an earlier legal position. This is especially important where a practice has evolved gradually.
Customary international law can develop. A practice may become more widespread, legal views may become clearer, and conduct previously treated as emerging may acquire established customary status. The opposite possibility also exists: an asserted rule may fail to mature because the necessary practice or opinio juris does not develop. Political change alone does not alter customary law. The legal rule changes only when the relevant evidence demonstrates the necessary development.
The relationship between custom and treaties must be kept precise. A treaty provision may codify a customary rule that already existed, may contribute to the crystallisation or development of an emerging rule, or may establish obligations that arise solely from the treaty relationship. These are different legal situations. The presence of a provision in an important multilateral convention does not automatically establish that an identical customary rule exists.
This distinction matters when a State is not party to the relevant treaty. If an obligation exists only because of treaty consent, the non-party is not bound by that treaty provision merely because the convention is widely accepted. If the same obligation independently exists as customary international law, however, its legal basis is different. The Court must therefore identify whether the alleged obligation is treaty-based, customary, or supported by both sources.
Treaty participation and treaty compliance must also be treated carefully as evidence. A State complying with a treaty may simply be performing its treaty obligation. That conduct does not automatically demonstrate an independent customary obligation. Evidence from States outside the treaty relationship, together with legal statements concerning the character of the rule, may therefore be important when the customary status of the provision is disputed.
The distinction between codification and progressive development is equally important. A treaty may record a rule that States already regard as existing law, or it may formulate a rule intended to develop international law. The treaty text alone cannot always answer which situation exists. The Court must examine the circumstances, State positions, practice and other relevant evidence to determine whether the provision reflects existing custom, contributes to its formation or remains a treaty rule.
International organisations and their resolutions can contribute relevant evidence but do not receive automatic customary status. Resolutions may record State positions, express legal principles, contribute to the development of legal views or provide evidence of opinio juris. Their significance depends upon the circumstances of adoption, the participation and explanations of States, the language used and subsequent practice. Political agreement on a resolution is not automatically equivalent to universal acceptance of every proposition in it as law.
National legislation can be evidence of State practice and sometimes of a State's legal understanding. Its significance depends upon why the legislation was adopted and how it relates to international law. A domestic measure enacted for purely internal administrative purposes may have limited value in proving an international legal obligation. A measure expressly adopted to implement an international legal requirement may provide stronger evidence concerning the State's understanding, although the Court must still consider the wider international record.
National judicial decisions may likewise reveal how a State's legal institutions understand international law. A domestic judgment expressly analysing customary international law can be useful evidence of the State's legal position. It remains evidence concerning one State, however, and one domestic judgment does not automatically establish a general international rule. Its significance must therefore be assessed alongside the practice and legal positions of other States.
Military manuals and official instructions can be especially important in fields involving armed conflict. They may show the standards a State instructs its forces to observe and may therefore reveal both expected conduct and legal understanding. Their evidential value still depends upon context, authority, consistency and relationship to actual State conduct. An instruction may express an aspiration or operational policy rather than necessarily proving the existence of a general customary rule.
International judicial decisions have an important subsidiary role in identifying and explaining custom. Courts can organise extensive evidence, assess competing State positions and articulate persuasive legal reasoning. The fact that a court has previously described a proposition as customary is not, however, a substitute for examining the legal foundation of that conclusion where the customary status remains disputed. The Court must distinguish the persuasive value of judicial reasoning from the underlying evidence establishing the rule.
Scholarly writings have a similar subsidiary function. Academic works can collect State practice, identify competing interpretations, explain judicial decisions and help the Court locate relevant evidence. Scholarship does not itself create customary international law. The Court should therefore use scholarly analysis as an aid to legal reasoning while grounding the ultimate finding in recognised sources and the evidence required by the customary-law test.
The Court must also distinguish custom from general principles of law. Both are recognised sources of international law, but their legal foundations differ. If a party relies upon both, the Court should explain separately why each source supports the proposed legal conclusion. The existence of one source should not simply be assumed to establish the other.
The scope and content of a customary rule must be identified with precision. Establishing a general principle does not automatically establish every detailed obligation, exception, threshold or consequence that a party may seek to derive from it. The Court should formulate the alleged rule precisely and test the evidence against that formulation. Where evidence establishes only part of a proposed rule, the Court should not expand the legal conclusion beyond what the evidence supports.
The same principle applies to exceptions. If a party claims that a customary rule contains an exception, the existence and scope of that exception require their own legal analysis. Evidence establishing a general rule does not automatically establish every claimed exception. The Court should identify the legal basis relied upon and examine the relevant practice, opinio juris and contrary positions.
Contrary State practice must be considered honestly and completely. Evidence supporting the alleged rule should not be selected while inconvenient material is ignored. A State may violate a rule, claim an exception, dispute its scope or reject the rule itself. The legal significance of the conduct depends upon the State's explanation and the responses of other States. A sound judgment should address material contrary evidence rather than conceal uncertainty.
The quality of evidence is therefore as important as its quantity. Ten ambiguous examples may be less persuasive than a smaller body of authoritative, consistent and legally reasoned practice. The Court should consider who acted, what was done, why it was done, how the State described its conduct, how other States responded and whether the evidence concerns the precise rule alleged.
The evidential record should also be organised chronologically where development over time matters. Early practice may show the emergence of a proposition, while later practice may demonstrate whether States came to accept it as law. Later developments should not be used mechanically to establish an earlier rule. The Court must keep the temporal question visible throughout its reasoning.
A customary-law finding should therefore contain a clear chain of reasoning. The Court should identify the precise rule alleged, define its geographical and temporal scope, identify the relevant States, collect and classify the evidence of State practice, assess the evidence of opinio juris, examine contrary practice and objections, consider the proper role of treaties and other authorities, and finally determine whether the combined evidence establishes the rule.
This method protects against two opposite errors. The first is treating political preference, repeated behaviour or institutional language as law without sufficient evidence. The second is treating custom as impossible merely because no treaty expressly contains the alleged rule. Customary international law is an independent source of international law and must be assessed through its own legal requirements.
For the proposed Court, this disciplined approach is essential. A judgment should distinguish findings of fact from findings about State legal positions and from conclusions of law. It should identify why particular evidence has been given weight and why contrary material has or has not affected the conclusion. Where the evidence is insufficient, the Court should say so rather than manufacture certainty.
The definition of customary international law can therefore be stated precisely: it is a rule of international law established through sufficiently relevant State practice accompanied by opinio juris. The rule must be identified with sufficient precision that the evidence can be tested against its content, scope and temporal application.
The definition is not a mechanical formula. It is a framework for legal reasoning. The Court does not simply count States, collect quotations or repeat earlier judgments. It evaluates evidence according to the legal proposition being established and determines whether the complete record supports the asserted customary rule.
The next sections build on this foundation rather than repeating it. Section 3 will examine State practice in greater depth, including its forms, evidential weight, generality and consistency. Section 4 will examine opinio juris and the evidence of legal acceptance. Section 5 will address the formation and development of custom, including the significance of time, specially affected States and persistent objection. Section 6 will bring the evidence together and examine how competing and contrary material should be evaluated.
The student should therefore understand Section 2 as establishing the legal definition and structure of the doctrine. The objective is not memorisation of the phrase “State practice plus opinio juris”. The objective is understanding what those elements mean, why both are required, how the alleged rule must be formulated, how evidence must be evaluated, and why the Court must distinguish customary obligation from treaty obligation, political preference, habit and judicial assertion.
That understanding provides the foundation for the later sections, where each component will be examined with greater evidential and legal detail and then applied to contested international disputes.
Article 38 of the Statute of the International Court of Justice provides the classic starting formulation by referring to international custom as evidence of a general practice accepted as law. Its wording therefore directs the Court to three connected questions: what practice exists, whether that practice is sufficiently general and representative, and whether it is accepted as law. Article 38 supplies the legal framework; the evidence must still establish the particular rule alleged.
The phrase “general practice” does not require identical conduct by every State. The Court considers whether the practice is sufficiently widespread and representative for the particular rule claimed, including the relevance of States particularly concerned with the subject matter. The quality, consistency and legal explanation of the practice matter alongside its numerical extent.
“Accepted as law” prevents frequency from becoming a substitute for legal obligation. Repeated conduct may result from convenience, domestic arrangements or reciprocity. Conversely, an official statement, diplomatic protest or legal submission may expressly identify conduct as required or permitted by international law. The Court therefore examines both what States do and the legal character they attribute to that conduct.
National legislation and national judicial decisions illustrate this evidential approach. Domestic legislation may demonstrate State practice, but its significance depends upon why it was enacted and whether it expresses an international legal position. A national judgment may reveal how a State's legal institutions understand a customary rule, but one domestic decision does not itself create a general international rule. Each must be assessed within the wider international record.
A useful illustration is the North Sea Continental Shelf cases before the International Court of Justice. Germany, Denmark and the Netherlands were involved in disputes concerning the delimitation of continental shelf areas in the North Sea. Denmark and the Netherlands relied upon the equidistance principle associated with Article 6 of the 1958 Geneva Convention on the Continental Shelf, while Germany was not bound by that provision in the same treaty relationship. The Court therefore had to consider, among other issues, whether the equidistance principle had independently become a rule of customary international law.
The significance of the case lies in the Court's refusal to treat treaty language as automatically establishing an identical customary rule. The Court examined whether the treaty provision reflected an existing rule, whether subsequent practice was sufficient to produce a customary rule and whether the necessary acceptance as law could be demonstrated. The case therefore shows the student how a court separates three questions that can otherwise become confused: what the treaty requires of its parties, what States have actually done, and whether the evidence establishes an independent customary obligation.
The case also demonstrates why the formation of custom must be tied to evidence rather than assumption. A rule does not become customary simply because it appears reasonable, desirable or widely discussed. Nor does every treaty provision become custom because many States subsequently comply with it. Where compliance may be explained by treaty obligation, the Court must ask what evidence demonstrates an independent legal acceptance beyond that treaty relationship.
This approach is equally important when a customary rule is alleged to have developed from an emerging practice. The Court must identify the earlier position, the subsequent conduct and the legal explanations accompanying that development. A later rule cannot simply be projected backwards to a period when the necessary practice and opinio juris had not yet developed. Conversely, a developing rule should not be dismissed merely because it was not universally established at its earliest stage. The legal question is whether the evidence at the relevant time supports the claimed status of the rule.
State consent requires similar precision. Customary international law is not based upon the same form of express consent as a treaty, but this does not mean that custom is unrelated to States. State practice and opinio juris are themselves evidence of how States behave and understand their legal obligations. A State's lack of treaty participation may therefore be significant when determining whether a treaty obligation applies, but it does not by itself answer whether an independent customary obligation exists.
The distinction becomes particularly important where a treaty and customary rule contain similar wording. The same conduct may be required by a treaty for one State and by custom for another, while a third State may be bound by both. The Court should identify each legal basis separately because the consequences of breach, interpretation and applicable exceptions may depend upon the source of the obligation. Similar wording does not necessarily mean identical legal foundations.
The Court must also distinguish between proving the existence of a rule and proving its precise content. Evidence may establish a general customary principle while leaving uncertainty over its scope, exceptions or detailed requirements. A judicial finding should therefore state the rule no more broadly than the evidence permits. If the evidence establishes that States accept a general prohibition but does not establish a particular threshold or exception, the Court should not invent the missing detail.
This is particularly important in cases involving serious consequences. Parties may rely upon a broad statement of customary law and then ask the Court to derive a specific prohibition, permission or duty from it. The Court should separate those propositions and test each against the evidence. A conclusion that a general customary principle exists does not automatically answer every question that may arise from it.
The definition of customary international law is therefore both substantive and methodological. Substantively, it is a recognised source of international law independent of any single treaty relationship. Methodologically, its existence is established through assessment of State practice and opinio juris, together with the appropriate consideration of the scope, period, affected States and competing evidence. Judicial decisions, resolutions, legislation, treaty materials and scholarship can assist the inquiry, but their value depends upon what they actually demonstrate.
The central judicial discipline is consequently to move from evidence to legal conclusion without skipping the intervening reasoning. The Court should be able to show which evidence establishes practice, which evidence demonstrates legal acceptance, why the practice is sufficiently general and representative, how contrary evidence has been treated, and why the resulting rule has the precise scope stated in the judgment. That is the foundation on which the later sections will build their detailed treatment of practice, opinio juris, formation and evidential assessment.
State practice is one of the two constituent elements used to identify customary international law. It concerns conduct and official acts attributable to States and the evidence showing how States behave and express their legal positions. The Court's task is not simply to collect repeated behaviour. It must determine whether the conduct is attributable to the State, relevant to the precise rule alleged, sufficiently general and representative, sufficiently consistent, and capable of supporting the customary rule when considered together with the required opinio juris.
State practice has no single physical form. It may consist of physical acts, diplomatic correspondence, official statements, legislation, executive measures, administrative decisions, military manuals and instructions, national judicial decisions, treaty-related conduct, conduct during negotiations, participation in international organisations and responses to international incidents. The relevant forms depend upon the subject matter. Evidence concerning maritime law may differ from evidence concerning diplomatic immunity, armed conflict, environmental protection or jurisdiction. The Court should therefore identify the precise legal proposition before deciding what evidence is relevant.
Actual conduct can be important evidence. A State exercising jurisdiction over foreign vessels, administering territory, enforcing a border, conducting military operations or responding to an international incident may demonstrate its position through action. The same physical act can nevertheless have different legal significance depending upon its circumstances and the explanation given by the State. A State may act because it considers conduct legally required, legally permitted, politically desirable or practically necessary. The Court must investigate that distinction rather than assume the reason.
Official statements may reveal a State's legal position. Heads of government, foreign ministers, ambassadors and authorised representatives may explain claimed rights, deny another State's entitlement, describe conduct as lawful or unlawful, or identify the legal basis for action. The Court should consider the authority of the speaker, the precision of the statement, its circumstances and its consistency with other governmental positions.
Diplomatic correspondence is particularly useful because it often records legal positions in response to concrete events. Diplomatic notes, protests, claims, replies and formal communications may identify the rule a State believes applies. A legal protest should be distinguished from a political complaint. A State may condemn conduct as dangerous or unacceptable without asserting that international law prohibits it. A communication expressly identifying an international legal obligation or entitlement carries different evidential significance.
National legislation can constitute State practice because it is an official act of the State. It may be relevant to jurisdiction, nationality, immunity, maritime affairs, territorial control or treatment of foreign persons. Its weight depends upon what the legislation does and why it was adopted. A domestic rule adopted for internal administrative convenience does not automatically establish an international legal position. Legislation expressly presented as implementing an international obligation may be more informative, but still has to be assessed alongside the wider international record.
National judicial decisions can likewise provide evidence. A domestic court may apply international law, recognise a customary rule, reject an asserted international entitlement or explain the State's understanding of an obligation. Such a decision demonstrates a legal position within that State's institutions; it does not, simply because the judgment calls a rule “customary”, establish the rule generally. Its value must be assessed with other States' practice and legal positions.
Administrative practice may supplement legislation and judicial decisions. Government departments may repeatedly apply approaches concerning foreign vessels, diplomatic personnel, extradition, jurisdiction, border control or international claims. The Court should consider whether the conduct represents an established governmental position or an isolated administrative error and whether the officials involved possessed the relevant authority.
Military manuals, operational instructions, rules of engagement and official training materials may be especially important in questions concerning armed conflict. They can reveal what a State officially instructs its armed forces to do and what legal standards it considers applicable. Actual military conduct can provide additional evidence, but an isolated act by an individual member of the armed forces does not automatically represent State practice. Attribution, official policy and subsequent governmental explanations must be examined.
Attribution is therefore fundamental. The Court is concerned with conduct representing the State, not every act performed by a person connected with it. It should consider the status and authority of the actor, whether the conduct occurred in an official capacity, whether the government subsequently adopted or repudiated it and whether it forms part of an identifiable governmental position. Subsequent governmental conduct can clarify an earlier event. Adoption, repudiation or official explanation does not erase the event but may materially affect its evidential weight.
The different forms of practice are not interchangeable. A military manual may demonstrate the rule a State instructs its forces to observe, while battlefield conduct demonstrates what occurred. A diplomatic protest may demonstrate a legal objection, while legislation may demonstrate an official domestic measure. Each source must be assessed according to the proposition it can actually support.
Treaty-related conduct requires particular care. When a State implements a treaty provision, the conduct may simply demonstrate compliance with a treaty obligation. It does not automatically demonstrate an independent customary obligation. Treaty negotiations can also provide evidence where States describe a provision as existing law, reject that description or present it as progressive development. The Court must identify what the conduct or statement actually establishes.
Participation in international organisations is another source of evidence. States make statements, submit proposals, vote, explain votes and respond to institutional proceedings. These acts may reveal State practice or legal position. The conduct of States must, however, be distinguished from the conduct of the organisation itself. A resolution may be important evidence without itself being identical to State practice.
Voting behaviour requires context. A State may support a resolution for political reasons while disagreeing with its legal implications. It may instead explain that its vote reflects acceptance of an existing legal principle. The Court should therefore consider the resolution, explanations of vote, reservations or qualifications, statements before and after adoption and subsequent State conduct.
International incidents can produce concentrated evidence. When a military, maritime, diplomatic or other incident occurs, States may justify their conduct, issue protests, make legal claims and respond to objections. The sequence can reveal competing legal positions. The Court should examine each part of the exchange rather than treating the incident as a single undifferentiated example.
Maritime and territorial practice demonstrate why precision is essential. Conduct concerning navigation, passage, fisheries, maritime enforcement, jurisdiction or maritime boundaries may appear similar while relating to different legal rules. Exercise of governmental authority, border control and administration may be relevant in territorial disputes, but the same conduct may be interpreted differently by opposing parties. The Court must identify the precise proposition and assess the legal claim accompanying the conduct.
State practice may be positive or negative. Positive practice consists of conduct undertaken by States. Negative practice may involve refraining from conduct where a legal response might have been expected. Negative practice is difficult to interpret because inactivity may result from ignorance, lack of opportunity, lack of interest or political calculation.
Silence is therefore not automatically acquiescence. Where a State knows of conduct affecting its legal interests, has an opportunity to respond and circumstances are such that a response could reasonably be expected, silence may become relevant evidence. The inference must still be supported by the surrounding facts. The Court should not convert every absence of protest into acceptance of a customary rule.
Generality concerns the breadth of practice. A general customary rule cannot ordinarily be established from the conduct of one or two States alone. The Court considers geographical distribution, the number and diversity of States involved and whether the practice represents the relevant international community. Generality does not require mathematical universality, but the evidence must be sufficiently broad for the rule claimed.
Representativeness requires attention to States whose interests and activities are particularly connected with the subject. In specialised fields, specially affected States may have greater opportunities to demonstrate relevant practice. Their evidence can therefore be particularly informative. It is not automatically decisive, and political, military or economic power does not itself determine legal weight.
Consistency is also important. Substantially contradictory practice may fail to establish a clear rule, but absolute uniformity is not required. A State may breach an accepted rule while continuing to recognise it, claim an exception or dispute only its application to particular circumstances. The Court must therefore examine the explanation accompanying contrary conduct.
A breach can sometimes support rather than disprove the existence of a rule. If a State acknowledges a prohibition but acts contrary to it, the conduct may demonstrate breach rather than rejection. The State's explanation, protests from other States and subsequent governmental position help determine whether the conduct represents breach, exception, rejection or uncertainty.
The quality of evidence matters as much as quantity. Ten sources reproducing one original diplomatic note are not ten independent examples of State practice. Conversely, a smaller body of authoritative, consistent and legally reasoned material may be more persuasive. The Court should identify original sources and avoid double-counting derivative material.
Duration is relevant but there is no universal minimum number of years required before custom can exist. A rule may develop over a long period or, in appropriate circumstances, relatively rapidly. The Court should examine intensity, consistency, breadth and legal character rather than substitute an artificial time requirement for legal analysis.
Temporal analysis becomes essential where the dispute concerns whether a rule existed at a particular date. Later practice may demonstrate subsequent development without proving that the rule existed earlier. The Court should distinguish evidence available during the relevant period from later developments and avoid projecting a mature rule backwards without justification.
Technological change may create new circumstances without automatically creating new law. New methods of navigation, communication, surveillance or military activity may instead involve the application of existing rules to new facts. The Court should determine whether States are adapting an existing rule or developing a genuinely distinct customary rule.
Exceptional circumstances require similar care. War, natural disasters, economic crises and other emergencies may produce conduct that is not representative of ordinary practice. The Court should examine whether States present the conduct as exceptional or as an application of a general legal rule.
The legal explanation accompanying conduct is consequently central. States may act because they believe international law requires or permits the conduct, or because the conduct is politically, economically or strategically desirable. The physical act may look identical in each case. Its evidential significance depends upon the legal and factual context.
Reciprocity can also explain repeated conduct. States may follow the same practice because they expect equivalent treatment from one another. Such reciprocal behaviour may be stable and widespread without establishing an independently binding customary rule. The Court should ask whether the conduct is regarded as legally required beyond the reciprocal arrangement.
State claims form part of the evidential record. A State may repeatedly assert sovereignty, jurisdiction, immunity, resource rights or another entitlement. Claims accompanied by corresponding conduct may be significant, but unilateral assertion does not establish a general customary rule. The responses of other States—acceptance, objection, qualification or legally significant silence—must also be considered.
International litigation submissions may reveal detailed legal positions. A State may make extensive arguments concerning customary law before an international tribunal. Those submissions provide evidence of what that State claims the law to be, but advocacy in litigation should not automatically be treated as independent proof of general acceptance.
The Court should distinguish independent from derivative evidence. It should ask who produced the material, what authority that actor possessed, when it occurred, what conduct or position it records, the circumstances and the precise proposition it supports. This classification prevents a large collection of related material from appearing stronger than its underlying evidence.
A mixed evidential record must be analysed rather than averaged. Imagine that fifteen States have legislation consistent with an alleged rule, ten have made supporting diplomatic statements, five expressly reject it and several remain silent. The Court cannot simply conclude that fifteen plus ten outweigh five. It must ask why the legislation was adopted, whether the statements express legal positions, why the five States reject the rule, whether the silent States knew of the practice and whether the States involved are representative.
If several of the fifteen States are parties to a treaty containing the same rule, their conduct may be explained by treaty obligation rather than independent custom. If non-parties also follow the rule and expressly state that international law requires it, the customary claim becomes stronger. This demonstrates why legal explanation is essential and why treaty compliance cannot automatically be counted as customary practice.
A particularly important judicial example is the International Court of Justice's judgment in Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States). The Court examined customary rules concerning the use of force and non-intervention and considered State practice and legal positions alongside treaty law. The case demonstrates that corresponding treaty provisions do not prevent a court from examining whether independent customary obligations exist.
The Nicaragua case also illustrates the treatment of contrary conduct. Conduct inconsistent with an asserted rule does not automatically disprove the rule. The Court must consider whether the State rejects the rule, claims an exception, justifies its conduct on another legal basis or accepts the general rule while disputing its application. The reactions and legal positions of other States form part of that assessment.
The case therefore provides a practical model for judicial reasoning. The Court does not ask only whether States have acted consistently in a physical sense. It examines the broader pattern of conduct, official explanations and legal positions and determines whether the evidence supports the customary proposition being asserted. The student should understand the case as an example of method rather than as a substitute for the underlying evidence.
The Court should also distinguish evidence concerning the existence of a rule from evidence concerning its content. Practice may establish a general principle while leaving uncertainty about its detailed requirements or exceptions. Evidence establishing a prohibition does not automatically establish every threshold, procedural requirement or exception that a party seeks to derive from it. The Court should formulate the rule precisely enough that the evidence can actually be tested against it.
Independence of practice also matters. Conduct undertaken solely because of a specific treaty relationship or required by another binding obligation may have different evidential significance from conduct adopted as an independent expression of the State's legal position. Independence does not mean States must act without reference to one another; it means the legal reason for the conduct must be understood.
For judicial reliability, significant evidence should be traceable from source to proposition. If legislation is relied upon, the Court should identify the legislation, its purpose and the proposition it supports. If a diplomatic protest is relied upon, the legal objection and context should be clear. If contrary conduct is treated as breach rather than rejection, the judgment should explain why.
This traceability is especially important where a customary finding may later be applied in proceedings with serious consequences. The Court must not create apparent certainty by accumulating references without explaining their legal significance. Another judge should be able to understand the evidential path from source, through assessment, to legal conclusion.
The proper method is therefore systematic. First identify the precise rule alleged. Second identify the relevant States and period. Third collect attributable practice. Fourth classify each item by source, authority and context. Fifth assess breadth, representativeness and consistency. Sixth consider contrary conduct, protests and legally significant silence. Seventh distinguish treaty compliance, political policy, reciprocity and other explanations. Finally determine what the practice actually establishes, while leaving the separate assessment of opinio juris to the next stage.
State practice is consequently not a catalogue of government behaviour to be counted mechanically. It is an organised body of evidence whose legal significance depends upon attribution, relevance, context, generality, representativeness, consistency, time, geographical scope and the legal explanations accompanying conduct.
For the proposed Court, a finding based upon State practice should identify the material relied upon, explain its evidential weight, address material contrary evidence and state clearly what the evidence does not establish. Where the record is insufficient, the Court should record that limitation rather than manufacture certainty.
The essential questions are therefore: whose conduct is it, what happened, when did it happen, why did the State act, what legal position accompanied the conduct, how representative and consistent is the practice, what contrary evidence exists, and what precise proposition does the evidence establish?
Even strong State practice does not by itself establish customary international law. The Court must still determine whether the practice is accompanied by opinio juris. Section 4 will examine that second constituent element and the evidence required to distinguish legal conviction from habit, convenience or political preference.
A court applying this method should be able to demonstrate its reasoning through a worked evidential assessment. Suppose the alleged rule concerns a restriction on a particular form of State conduct. The evidence contains official legislation from several States, diplomatic statements from others, actual conduct by a smaller number of States, contrary protests from several States and silence from others. The Court should first separate the evidence by source and ask what each item proves. Legislation may demonstrate an official domestic position, actual conduct may demonstrate what States have done, and diplomatic protests may demonstrate that particular States regard the conduct as legally prohibited. None of these categories should automatically be treated as conclusive.
The Court should next test whether the evidence is independent. If several States adopted identical legislation because they were implementing the same treaty obligation, the apparent breadth of the practice may be less significant for a claim of independent custom. If other States adopted similar measures without that treaty relationship and expressly described the rule as required by international law, that evidence may carry greater weight. The Court should record the distinction rather than simply count all of the legislation together.
Contrary practice should then be examined on its own terms. If a State acts against the alleged rule but simultaneously protests that its conduct is lawful only because of a claimed exception, the evidence may concern the scope of the rule or the existence of the exception rather than rejection of the rule itself. If the State instead expressly states that no such customary obligation exists, that is a materially different position. The Court should identify the difference and explain its effect on the overall assessment.
Silence should be considered only where its circumstances make it legally informative. If a State was unaware of the conduct, had no practical opportunity to respond or had no identifiable legal interest requiring a response, silence may add little. If the State was directly affected, clearly aware of the conduct and had repeatedly engaged in the relevant diplomatic exchange, silence may carry greater evidential significance. Even then, the Court should explain the inference rather than presume acquiescence.
The Court should finally consider the quality and representativeness of the record. A large number of ambiguous examples may not establish the rule if the underlying reasons for the conduct are unclear. Conversely, a smaller body of authoritative practice accompanied by clear legal positions may be persuasive. Specially affected States may be particularly informative in a specialised field, but their positions must still be considered within the broader international record.
The resulting judicial conclusion should therefore identify both what the evidence establishes and what it does not establish. For example, the Court may conclude that a broad pattern of State practice exists but that the record does not establish the precise exception claimed by a party. Alternatively, the Court may find that practice is widespread but too inconsistent, or that much of the apparent practice is treaty-driven and therefore insufficient to establish an independent customary rule. This form of conclusion is preferable to an artificial yes-or-no assessment of an evidential record containing different degrees of legal significance.
Opinio juris is the second constituent element used in identifying customary international law. It concerns the acceptance by States that a particular practice is legally required, legally permitted or otherwise recognised as law. State practice concerns what States do and what official conduct they undertake; opinio juris concerns the legal character States attach to that conduct. The two elements interact, but neither should simply be treated as proof of the other.
The distinction is essential because States engage in countless forms of international conduct without believing that international law requires or permits the conduct. Governments may follow diplomatic traditions, adopt similar policies because they are efficient, behave reciprocally, follow domestic practice, respond to political pressure or act for economic and strategic reasons. Repetition alone therefore does not establish a customary legal rule.
Opinio juris is not a test of the private thoughts of individual officials. A court assesses the legal understanding of States objectively through available evidence. That evidence may include official statements, diplomatic correspondence and protests, legal submissions, legislation, national judicial decisions, military manuals, treaty negotiations, explanations of votes and other authoritative governmental material.
The Court should distinguish three possible legal understandings. A State may believe international law requires conduct; it may believe international law permits conduct; or it may recognise a legal entitlement, status or proposition without necessarily asserting a universal obligation. These are different propositions. The Court must identify precisely which legal proposition is alleged to have customary status.
The most direct evidence occurs when a State expressly identifies the legal basis for its conduct. A government may state that international law requires a particular action, that another State's conduct violates international law, or that an entitlement is recognised by customary law. Such statements can be important evidence, but their authority, context, consistency and relationship to actual State practice must be examined.
Diplomatic protests are particularly valuable where they respond to concrete international events. A protest may show that a State regards conduct as legally wrongful rather than merely politically undesirable. The Court should examine its language, circumstances, authority and continuity. A political complaint is different: condemnation of conduct as dangerous, offensive or undesirable does not necessarily demonstrate belief in a legal rule.
Silence requires even greater caution. Failure to protest does not automatically establish opinio juris. Silence may result from lack of knowledge, lack of opportunity, diplomatic restraint, uncertainty or lack of sufficient interest. It may become legally informative where a State knows of the conduct, has a genuine opportunity to respond, has a relevant legal interest and circumstances are such that a response could reasonably be expected. The inference must be explained rather than presumed.
National legislation may contribute evidence of opinio juris. A statute may implement a treaty, pursue domestic policy or reflect an independent understanding of international law. If the government or legislature expressly states that the measure is required by customary international law, that explanation may be significant. If no such explanation exists, the legislation may demonstrate State practice without necessarily proving the required legal conviction.
National judicial decisions can similarly reveal a State's legal understanding. A domestic court may determine that a rule forms part of international law, distinguish treaty obligations from customary obligations or explain why conduct is legally required. The Court should consider the authority of the domestic tribunal, its reasoning and whether other State institutions accept or reject the position. A domestic judgment does not itself create a general customary rule.
Military manuals, rules of engagement and legal guidance can be particularly important in armed-conflict questions. They may show what a State instructs its armed forces they are legally required to do. Their evidential value depends upon their authority, date, wording, consistency with other official positions and relationship to actual practice. The Court must determine whether the material reflects treaty obligations, domestic policy or an asserted customary rule.
Positions taken before international organisations can also demonstrate opinio juris. States may make formal statements, explain votes, propose language or participate in negotiations by expressly invoking international law. A resolution adopted by an organisation is not automatically customary law. The Court must consider the positions of States, the circumstances of adoption, the legal language used, explanations of vote and subsequent conduct.
Treaty practice requires particular care. A State complying with a treaty provision is acting under a treaty relationship, and that conduct does not automatically demonstrate independent customary obligation. The Court should distinguish at least three possibilities: the State complies because the treaty binds it; it regards the provision as declaratory of an existing customary rule; or it accepts the provision as a new contractual commitment without considering it already customary.
Treaty negotiations can clarify the distinction. A State may state that a provision codifies existing customary law, may argue that it represents progressive development, or may accept compromise language without taking a position on customary status. These positions have different evidential significance. A proposal for future law should not automatically be treated as proof that the proposed rule already exists.
The same caution applies to declarations and resolutions adopted for political purposes. States may support humanitarian, environmental, economic or diplomatic objectives because they consider them desirable without believing every element is legally binding. Political consensus can be important evidence of international attitudes, but the Court must determine whether States themselves attribute legal character to the relevant proposition.
Multiple motives do not necessarily defeat opinio juris. A State may consider conduct legally required while also finding it politically useful, morally desirable or economically advantageous. The relevant question is whether the evidence demonstrates that the State also regards the conduct as required, permitted or recognised by international law.
Reciprocity illustrates the problem. States may behave alike because they expect equivalent treatment. Reciprocal practice can be widespread and stable without establishing an independently binding customary rule. The Court should therefore ask whether the legal understanding extends beyond the reciprocal arrangement.
Habit and convenience present the same difficulty. A government may consistently follow a practice because its administrative system makes that approach efficient. Several States may do the same for practical reasons. Uniform conduct is not enough unless the evidence demonstrates acceptance of the conduct as law.
Opinio juris can be supported by both positive and negative evidence. Positive evidence includes statements recognising legal obligations or permissions, protests alleging breach, legal submissions invoking customary rules and official explanations that conduct is required or permitted by international law. Negative evidence includes express rejection of the alleged rule, statements describing it as policy rather than law, reservations of legal position and explanations denying legal obligation.
Negative evidence must itself be interpreted precisely. A State may reject a broad formulation while accepting a narrower rule. It may reject an alleged exception while accepting the underlying prohibition. It may dispute the geographical or factual application of a rule without denying its existence. The Court must therefore formulate the alleged rule with sufficient precision before deciding what the evidence establishes.
Contradictory conduct does not automatically eliminate opinio juris. A State may breach a rule while continuing to acknowledge that the rule exists. The legal significance of the breach depends partly upon the State's explanation. If it expressly rejects the rule, that is evidence against the alleged legal acceptance. If it accepts the rule but claims an exception or disputes its application, the same conduct may instead confirm acceptance of the underlying rule.
This distinction between breach and rejection is fundamental. International law could not operate if every breach were treated as evidence that the rule no longer existed. The Court should determine whether the State claims that no rule binds it, or whether it accepts the rule while relying upon another legal argument.
Opinio juris can also be evidenced through international litigation. A State may invoke customary law in pleadings before an international tribunal. Such submissions reveal a legal position, but litigation advocacy must be assessed carefully. A position adopted only after proceedings begin may carry different weight from one consistently maintained through diplomacy, legislation, national judgments and international proceedings.
International judicial decisions have an important but distinct role. Courts organise and evaluate evidence of State practice and opinio juris and explain why a customary rule has or has not been established. The judicial decision itself is not a third constituent element. The Court must still identify the underlying State evidence.
The North Sea Continental Shelf cases illustrate this principle. The International Court of Justice considered whether the equidistance principle had become customary international law and examined practice together with the belief that the practice was legally obligatory. The Court did not treat inclusion of the principle in a treaty, combined with substantial participation in that treaty, as automatically establishing customary law. The case demonstrates that treaty participation cannot simply substitute for evidence of the constituent elements of custom.
The methodological lesson from North Sea is that widespread practice must be examined for its legal character. A court cannot reason that States follow a practice, therefore they must regard it as law, and then use that assumed legal character to prove the customary rule. The evidence supporting the inference must be identified.
The 2023 Nicaragua v. Colombia judgment provides a more recent example. The International Court of Justice examined customary international law concerning continental shelf entitlements and considered actual State practice together with opinio juris. The Court assessed the practice as a whole and found it sufficiently widespread and uniform, and its extent over time capable of expressing opinio juris.
That example demonstrates that opinio juris can sometimes be inferred from a sufficiently convincing body of practice when the circumstances justify the inference. The practice concerned a specialised legal question and States had repeated opportunities to make claims and objections. The important point is not that repeated practice automatically becomes opinio juris, but that the nature, consistency, duration and circumstances of the practice may provide evidence of legal acceptance.
The Nicaragua judgment also illustrates the importance of treaty context. Customary law may coexist with treaty rules concerning the same subject. A treaty provision can codify, clarify or develop customary law, but its customary status must still be assessed through the customary-law methodology rather than assumed from treaty participation alone.
A court should distinguish three propositions when evaluating evidence: first, that a State behaved in a particular way; second, that the State expressed a legal position concerning that behaviour; and third, that the wider evidence establishes a customary rule. The first concerns practice. The second is evidence relevant to opinio juris. The third is the judicial conclusion reached after the constituent elements have been assessed.
This distinction is especially important with emerging rules. States may declare that international law ought to develop in a particular direction without asserting that the proposed rule already exists. A statement supporting progressive development may therefore demonstrate an aspiration rather than existing customary law. Conversely, practice may become widespread before States clearly articulate its legal basis, requiring the Court to examine subsequent legal statements and other evidence.
The temporal question matters whenever the dispute concerns whether a rule existed at a particular date. Later statements may illuminate development but should not automatically be projected backwards. The Court should identify the evidence existing during the relevant period and determine whether customary formation had occurred by that time.
The Court should also distinguish the existence of a rule from its content. States may accept a general principle while disagreeing about exceptions, thresholds, procedures or geographical scope. Such disagreement does not necessarily demonstrate absence of opinio juris. It may instead show that legal acceptance exists only at a narrower level than the rule asserted by a party.
A practical evidential assessment illustrates the method. Suppose twenty States follow a particular maritime practice, fifteen are parties to a treaty containing a similar provision, five non-parties also follow the practice, and only two non-parties expressly describe it as required by customary international law. Several treaty parties state that they act because the treaty binds them. The Court should not count all twenty States as equal evidence of opinio juris. Treaty compliance demonstrates practice, but the accompanying legal explanation may show that the conduct is treaty-based rather than independently customary.
If several treaty parties additionally state during negotiations and later diplomatic correspondence that the provision merely codifies existing customary law, their evidence becomes more significant. The legal explanation connects the practice to an independently recognised customary obligation. The Court must still consider contrary evidence, but it now has stronger grounds for finding opinio juris than it would have from treaty compliance alone.
A second example concerns a proposed prohibition. States generally refrain from conduct. Some expressly state that international law prohibits it. Others avoid the conduct because it is politically undesirable. Others say that international law does not prohibit it but that they have no present reason to undertake it. The physical practice appears uniform, but the legal explanations differ. The Court cannot treat common abstention as sufficient proof of opinio juris without resolving the competing explanations.
A third example concerns a legal permission. States repeatedly permit an activity without a controlling treaty. Some describe it as an international legal right; others describe it as sovereign policy; affected States protest that it is unlawful. The Court may find widespread practice while concluding that opinio juris remains contested, or it may identify a narrower legal permission than the party asserted.
The Court must also guard against circular reasoning. It should not say that practice proves opinio juris merely because States followed the practice, and then use the supposed opinio juris to prove the legal significance of the practice. The Court must identify independent or sufficiently probative evidence of the legal character attached to the conduct.
Earlier judicial decisions should not be allowed to replace that analysis. A previous judgment may be authoritative as a statement of judicial reasoning, but customary law remains grounded in State practice and opinio juris. Where the evidence has developed or the issue is contested, the Court should return to the underlying evidence.
Scholarly writings have a related but different role. Academic commentary may locate evidence, explain competing interpretations and assist legal analysis, but scholarly opinion does not itself establish opinio juris. The Court should distinguish commentary about State legal positions from the State evidence itself.
International organisations require similar separation. A resolution may contain a strong statement of legal principle, but the Court must determine whether States supported it as law or merely supported a political objective. Explanations of vote, reservations, subsequent statements and conduct can reveal the difference. Unanimous voting does not remove the need to examine legal character.
The chronology of legal acceptance also matters. A State may initially describe a practice as policy and later recognise it as legally required. Another State may initially accept a rule and later challenge its scope. The Court should preserve that chronology and consider whether a position was maintained, changed or narrowed.
A reliable judicial framework therefore begins by formulating the precise customary proposition. The Court then identifies the relevant practice and the legal explanations accompanying it. It separates direct expressions of legal acceptance from conduct from which acceptance may be inferred. It examines supporting and contrary evidence, treaty and institutional material, the relevant period and the authority of the sources. Finally, it determines whether the evidence establishes acceptance as law.
The Court should record the limits of its inference. If the evidence establishes that States regard conduct as legally permissible but does not establish a general obligation, the judgment should say so. If it establishes a prohibition but not a claimed exception, those conclusions should be separated. If the evidence demonstrates an emerging legal position without completed custom, the Court should identify that distinction.
This method protects against three major errors: treating frequency as legality, treating legal terminology as conclusive without context, and treating political consensus as automatically equivalent to legal acceptance. Each error risks turning a factual or political observation into an unsupported legal conclusion.
For the proposed Court, this discipline is especially important where a customary finding may affect rights, liability or other serious legal consequences. The Court should identify the evidence, explain the inference and acknowledge genuine uncertainty. The absence of sufficient evidence should remain a legitimate judicial conclusion.
The central lesson is that opinio juris gives State practice its legal character. It does not require the Court to discover private intentions. It requires an objective assessment of authoritative evidence showing whether States understand the relevant practice as required, permitted or recognised by international law.
A complete judicial analysis should therefore allow another judge to follow the reasoning from source to conclusion: what the State did, what legal position it expressed, why that position is authoritative, what other States said or did, what contrary evidence exists, how treaty or political explanations affect the analysis, and why the totality of the evidence establishes or fails to establish legal acceptance.
Section 4 therefore provides the foundation for the next stage of customary-law analysis. The following section will bring State practice and opinio juris together and examine how the two constituent elements combine, including generality, consistency, duration, specially affected States, persistent objection and the precise content of a customary rule.
Where the evidence supports competing explanations, the Court should state those explanations explicitly before choosing between them. For example, if a State repeatedly follows a practice but expressly says that it does so as a matter of courtesy, the conduct remains relevant to State practice but provides weak evidence of opinio juris. If another State follows the same practice while repeatedly describing it as an international legal obligation, its conduct has a different evidential character. The Court should not treat the two States as having expressed the same legal position simply because their physical conduct is identical.
The Court should also consider whether a State's legal statement is connected to the conduct being examined. A general statement supporting respect for international law may be too broad to establish opinio juris concerning a particular customary rule. Conversely, a specific diplomatic protest identifying the precise obligation allegedly breached may provide much stronger evidence. The closer the legal statement is to the precise rule, the event and the conduct under examination, the more useful it may be in determining the State's legal understanding.
This approach also prevents a party from proving opinio juris by selecting isolated quotations. The Court should read important statements in their full context, compare them with the State's subsequent conduct and consider whether the position was maintained when circumstances changed. A legal position repeatedly expressed across different settings is generally more informative than a single unexplained statement. The Court should nevertheless assess each source on its own authority rather than assuming that repetition alone makes an assertion legally correct.
Formation of customary international law is the process by which State practice, accompanied by acceptance of that practice as law, becomes sufficiently established to constitute a customary rule. The inquiry is therefore concerned with two constituent elements: State practice and opinio juris. Neither should be treated as a simple substitute for the other. The Court must examine the evidence as a whole while maintaining the distinction between what States do and the legal character they attribute to what they do.
The first task is to formulate the alleged rule precisely. A party may describe a proposed rule in a sentence that actually contains several separate propositions: a prohibition, the circumstances in which it applies, an exception, a procedural requirement and a legal consequence. Evidence establishing one proposition does not automatically establish the others. The Court should identify who is said to be bound, during what period, in what circumstances and at what level of generality.
This precision is essential because evidence supporting a broad principle may not establish a narrower rule. States may agree that a general principle exists while disagreeing about its exceptions, thresholds, geographical scope or procedures. The Court should therefore test the precise legal proposition being asserted rather than silently enlarging a rule beyond the evidence.
Once the rule is formulated, the Court identifies relevant State practice. Practice includes official conduct and other authoritative manifestations of State behaviour relevant to the proposed rule. It must be assessed for breadth, consistency, representativeness and legal significance. Formation is not a numerical exercise. A court does not establish custom merely by counting States.
Generality concerns whether the practice is sufficiently widespread and representative for the type of rule claimed. It does not mean that every State must have expressly spoken. International law does not operate through a worldwide referendum. The Court instead examines the extent and character of participation and whether the relevant States accept the practice as law.
Representativeness is more than geographical arithmetic. Depending on the subject, the Court may need to consider States with particularly relevant interests or opportunities to engage in the practice. States directly involved in a specialised field may produce especially informative evidence. This is the significance of specially affected States: their practice may carry particular evidential weight because of their direct involvement, but it does not mean that powerful or prominent States automatically control the formation of custom.
Consistency concerns whether the practice is sufficiently coherent to support the proposed rule. Occasional departures do not necessarily prevent a rule from existing. A State may breach a rule while continuing to recognise the rule and attempting to justify its conduct through an exception or by disputing the rule's application to the facts. The Court must distinguish rejection of a rule from disagreement about its scope or application.
Contrary practice therefore requires explanation rather than simple counting. If a State acts inconsistently but says that its conduct is lawful because the alleged rule does not apply in the circumstances, the State may still accept the underlying rule. If it expressly states that no such customary obligation exists, the evidence points in a different direction. The Court should identify the legal explanation accompanying contrary conduct.
The same discipline applies to absence of practice. An incomplete evidential record is not necessarily evidence that the opposite rule exists. If few States have encountered the issue or had an opportunity to express a position, the Court may conclude that the evidence is insufficient rather than infer positive rejection. This distinction is particularly important in emerging areas of international law.
Duration may be relevant to the formation inquiry, but international law does not impose one universal minimum period for every customary rule. A rule may develop over a long historical period or, where the evidence is sufficiently extensive and convincing, over a comparatively short period. Rapid formation does not mean that evidence can be omitted. A short period makes careful examination of breadth, consistency, legal acceptance and representativeness especially important.
The Court should also distinguish temporary convergence from customary formation. States may respond in similar ways to a crisis, technological development or institutional problem because they face the same practical circumstances. Similar responses may disappear when the circumstances change. Unless the evidence shows that States regard the relevant conduct as legally required or permitted, temporary convergence should not automatically become customary law.
Emergency practice requires similar caution. States may adopt exceptional measures during war, economic crisis, natural disaster or other extraordinary circumstances. Such conduct may have limited value for ordinary circumstances if States themselves characterise it as exceptional. Conversely, repeated invocation of a legal exception may provide evidence about the existence and content of the underlying rule. The Court should examine how the States themselves characterise the conduct.
State practice must be considered together with opinio juris. A pattern of behaviour motivated only by convenience, political preference, habit, reciprocity or practical necessity does not automatically establish customary law. Conversely, a State may have several motives for acting and still regard the conduct as legally required or permitted. The relevant question is whether the evidence demonstrates a legal understanding in addition to any political or practical motive.
Evidence of opinio juris may include official statements, diplomatic correspondence and protests, legal submissions, legislation, national judicial decisions, military manuals, explanations of votes, treaty negotiations and other authoritative governmental material. The Court should assess each source for authority, context, relevance, consistency and connection to the precise rule alleged.
Diplomatic protests are particularly significant when they respond to concrete conduct. A protest may demonstrate that a State regards conduct as legally wrongful rather than merely undesirable. Silence can sometimes be relevant where a State knew of the conduct, had a genuine opportunity to respond, had a relevant legal interest and circumstances were such that a response could reasonably be expected. Silence should never be treated automatically as acceptance of law.
National legislation may demonstrate practice and, where accompanied by an authoritative legal explanation, may also provide evidence of opinio juris. National judicial decisions can reveal how State institutions understand international law, but a domestic judgment does not itself create a general customary rule. Military manuals and legal guidance may be particularly relevant in specialised fields, provided the Court determines whether they reflect treaty obligations, domestic policy or an asserted customary obligation.
International organisations provide important forums for States to express legal positions. Statements, explanations of votes, proposals, negotiations and responses to resolutions may therefore form part of the evidential record. But a resolution adopted by an international organisation is not automatically customary international law. The Court must distinguish the institutional act from the legal positions of the States participating in it and determine whether support for the text reflected legal acceptance or political agreement.
The same distinction applies to expert bodies and scholarly writings. Draft articles, reports and academic works can organise evidence, identify competing interpretations and assist the Court, but they do not replace State practice and opinio juris. The Court should identify whether such material reports existing law, proposes progressive development or offers an analytical conclusion.
Treaty law requires especially careful treatment. A treaty provision may codify an existing customary rule, contribute to the crystallisation of a developing rule, influence subsequent customary development, or create an obligation binding only upon its parties. Treaty participation by itself does not establish independent customary status.
If States comply with a treaty provision because the treaty binds them, that conduct demonstrates treaty practice but does not automatically prove that they accept the same rule independently as customary law. The evidence becomes more significant where States expressly describe the provision as reflecting existing customary law, maintain the position in relations involving non-parties, or otherwise demonstrate independent legal acceptance.
Codification must therefore be distinguished from crystallisation and progressive development. Codification records or formulates an existing customary rule in written form. Crystallisation concerns a developing rule that becomes sufficiently established during a particular period. Progressive development concerns the deliberate development of new legal standards through treaty-making or other processes where the proposed rule was not previously established in the same form. A proposal for future law is not automatically proof that the proposed rule already exists.
A treaty may bind a State on the basis of consent while the same rule independently binds that State through custom. A non-party may likewise be bound by a customary rule if the customary requirements are satisfied, but a treaty obligation cannot simply be extended to a non-party because the treaty has widespread participation.
Treaty negotiations and international conferences must be assessed in context. States may negotiate language to record existing law, develop new obligations, preserve disagreement or achieve political compromise. A statement that a provision codifies existing customary law is different from a statement that the provision represents desirable future law. The Court must determine which proposition the evidence supports.
The temporal dimension is central to formation. The Court should identify the date at which the alleged rule is said to have existed and reconstruct the evidence over time. A later acceptance of a rule does not automatically establish that it existed earlier. Later evidence may illuminate development or confirm an earlier position, but the Court must distinguish evidence of an already established rule from evidence that the rule developed subsequently.
Chronology is equally important for changes in State position. A government may initially describe a practice as policy and later recognise it as legally required. A State may initially accept a proposition and later narrow or reject it. The Court should examine whether positions were maintained, changed or qualified rather than selecting only the material most favourable to one side.
Persistent objection belongs within this chronological analysis. A State claiming persistent-objector status must have clearly objected while the customary rule was forming and maintained the objection. The Court should identify when the objection began, what proposition was rejected, how clearly the objection was expressed and whether it continued consistently.
A single political disagreement is not automatically persistent objection. Nor is an objection first made after a customary rule has become established necessarily sufficient to prevent the rule from applying. A State may also accept a general rule while objecting only to an exception, extension or particular application. The precise content and timing of the objection must therefore be established.
Regional and local custom require corresponding precision about the relevant community. Evidence outside the claimed regional or relational group may provide context but cannot automatically establish a regional or local rule. The Court must identify the States among whom the alleged custom operates and determine whether practice and legal acceptance are sufficiently established within that relationship.
Formation must also be distinguished from interpretation and application. Evidence establishing that a customary rule exists is not necessarily the same evidence needed to determine the rule's detailed meaning in a particular dispute. Similarly, evidence that conduct is unlawful does not automatically establish the precise remedy, jurisdictional consequence or individual responsibility arising from that conduct. Those questions may involve additional rules.
International judicial decisions have an important but distinct role. Courts identify, organise and assess State evidence and provide reasoned conclusions about the existence and scope of custom. Judicial recognition should not be confused with judicial creation. An earlier judgment may be highly persuasive, particularly where it carefully evaluates practice and opinio juris, but the later Court should still consider whether the underlying evidence, relevant period and precise rule are comparable.
Article 38 of the Statute of the International Court of Justice provides the familiar formulation of international custom as evidence of a general practice accepted as law. The formulation directs attention to the two constituent elements. It does not remove the need for the Court to examine the evidence in the particular case.
The formation inquiry should therefore guard against circular reasoning. The Court should not assume that repeated practice proves legal acceptance simply because it is repeated, and then use that assumed legal acceptance to establish the customary rule. It should identify evidence that gives the conduct its legal character. Conversely, legal statements should not be accepted without examining their context, authority, consistency and relationship to actual State conduct.
Formation is cumulative but not automatic. Evidence varies in quality. A large number of statements repeating an unsupported proposition may be less persuasive than a smaller body of authoritative governmental positions accompanied by consistent practice and reactions to contrary conduct. The Court should assess authenticity, authority, relevance, independence, consistency and proximity to the issue being decided.
The Court should also consider whether apparently separate evidence is genuinely independent. Several States may reproduce the same model law, treaty language or institutional standard. Such material remains relevant, but it should not automatically be treated as independent confirmation from every State. Conversely, independently developed practice accompanied by similar legal explanations may provide strong cumulative evidence.
A practical formation analysis can be illustrated by a hypothetical rule prohibiting a particular governmental measure. Suppose thirty States have legislation restricting the measure, twenty have issued statements describing it as unlawful, five expressly reject the alleged customary prohibition and several have not taken a position. The Court should not simply count thirty supporting States against five opposing States.
It should first determine whether the legislation represents independent State practice or implementation of treaty obligations. It should then examine the legal explanations accompanying the legislation and diplomatic statements. If ten States adopted their measures solely to comply with a treaty while the others independently described the prohibition as customary law, the evidential position is materially different from a simple numerical count.
The Court should then examine the five opposing States. It should ask whether they are particularly relevantly affected, whether their rejection has been consistent, whether they reject the existence of the rule or only its application, and how other States have responded. If the practice is concentrated in one region, the Court must also determine whether the claim is really one of general custom or regional custom.
This example demonstrates why formation requires both breadth and depth. Breadth concerns the distribution of relevant practice. Depth concerns the quality of the evidence, the legal explanations, consistency over time and the circumstances in which conduct occurs. Neither can be reduced to a single numerical threshold.
The Court should also distinguish evidence establishing existence from evidence establishing content. If the evidence establishes a general prohibition but does not establish the claimed exception, the Court should state those conclusions separately. It should formulate the customary rule no more broadly than the evidence permits.
If evidence remains insufficient, the Court should say so. Failure to establish a customary rule is not the same as establishing that the opposite rule exists. A judgment should not convert evidential uncertainty into a positive prohibition, permission or entitlement merely because one party has failed to prove its case.
This distinction is particularly important where customary findings may have serious legal consequences. The Court should not impose an international legal obligation merely because conduct is politically desirable, widely supported, morally persuasive or frequently repeated. It must establish the legal basis through evidence.
The final judicial finding should therefore make the evidential chain transparent. It should identify the proposed rule, relevant States, relevant period, State practice, opinio juris, treaty relationships, supporting material, contrary evidence, objections and the reasons why the combined record does or does not establish the rule.
A sound formation methodology can be stated as eight steps:
1. Formulate the precise proposed customary rule.
2. Identify the relevant States, circumstances and period.
3. Identify and classify the relevant State practice.
4. Assess breadth, consistency, representativeness and quality.
5. Identify and assess evidence of opinio juris.
6. Examine contrary practice, objections, exceptions and competing explanations.
7. Characterise treaties, institutional materials and judicial decisions according to their proper legal and evidential role.
8. State the conclusion at exactly the level supported by the evidence.
The method allows the Court to distinguish an established customary rule from an emerging principle, political expectation, temporary convergence or treaty obligation that has not acquired independent customary status. It also allows the Court to distinguish a general rule from a regional or local custom and an established rule from a proposed future development.
For the proposed Court, this methodology should become a standard judicial discipline. Relevant diplomatic material, official statements, national legislation and judgments, military materials, treaty practice, institutional positions and reactions to disputed conduct should be identified and preserved in a traceable evidential record. The purpose is not to collect documents indiscriminately, but to allow the reasoning from evidence to legal conclusion to be reviewed.
The formation of customary international law is therefore neither an automatic consequence of repeated conduct nor an exercise of judicial preference. It is a disciplined legal assessment in which State practice and opinio juris are examined together but remain conceptually distinct. Genuine customary development should be recognised when the evidence establishes it; unsupported claims should not be elevated into law.
The following section moves from formation to the evidential materials through which custom is identified and tested. It will examine how courts weigh different forms of evidence, distinguish primary evidence from supporting material and assess competing evidence without treating every international statement or document as having equal legal weight.
The Court should identify the point at which the evidence becomes sufficient to recognise formation rather than describing formation as an indefinite process. An emerging practice may contain strong supporting evidence without yet satisfying the customary requirements. The judgment should therefore distinguish evidence showing that a rule is developing from evidence establishing that the rule has already formed. Where the evidence crosses that threshold during a particular period, the Court should identify the relevant date or period and explain why the combination of practice and legal acceptance is sufficient at that stage.
This assessment should be made against the evidence as it existed at the relevant time. The Court should not decide formation by looking backwards from a rule that is now widely accepted and assuming that its present status proves its earlier existence. A later customary rule may have developed from earlier practice without having existed in its present form at every earlier stage. Establishing the date of formation is therefore part of accurate customary-law reasoning, particularly where the legal consequences of the rule depend upon whether it existed when particular conduct occurred.
The Court should also distinguish a rule that has formed from a rule whose content is still developing. States may accept the existence of a general obligation while continuing to disagree about its precise scope. In that situation, the Court may be able to recognise the established core of the rule while declining to recognise an additional disputed element. This avoids the false choice between accepting the entire formulation advanced by a party and rejecting the customary rule altogether.
The judicial decisions in the North Sea Continental Shelf cases provide a central illustration of formation analysis. The International Court of Justice was asked whether the equidistance principle contained in Article 6 of the 1958 Convention on the Continental Shelf had also become a rule of customary international law. The Court did not treat the existence of the treaty provision, or the number of States participating in the Convention, as sufficient by itself. It examined whether the relevant practice was sufficiently extensive and virtually uniform and whether States followed the practice from a sense of legal obligation. The case therefore demonstrates that formation requires an assessment of the customary elements independently of the existence of treaty language.
North Sea also demonstrates why the Court must investigate the character of State conduct. The fact that States had used an equidistance method in a number of maritime delimitations did not automatically establish that they considered themselves legally required to do so. The Court examined the circumstances of the practice and the reasons associated with it. For formation purposes, the lesson is that conduct must be connected to the legal proposition alleged. A pattern of behaviour may be evidence of an emerging rule without yet proving that the rule has formed.
The case also illustrates the importance of the period in which formation is alleged to have occurred. The Court was concerned with whether the customary rule had been established by the relevant time, rather than whether the principle might subsequently become customary. This reinforces the requirement to reconstruct practice and legal acceptance chronologically and to avoid using later developments as automatic proof of earlier law.
A more recent illustration appears in the 2023 Nicaragua v. Colombia judgment. The International Court of Justice considered the customary international law governing continental shelf entitlements and examined actual State practice together with opinio juris. In assessing the relevant practice, the Court considered the conduct and positions of States in relation to continental shelf claims and the responses to competing claims. It concluded that the relevant practice was sufficiently widespread and uniform and that its extent over time could be regarded as an expression of opinio juris. The judgment demonstrates how a court can move from individual items of State conduct to a conclusion about the formation and existence of a customary rule when the evidence, considered as a whole, satisfies the required standard.
The Nicaragua judgment is also useful because it demonstrates the importance of identifying the precise customary proposition. The Court did not simply ask whether States generally accepted principles concerning continental shelves. It examined the specific legal entitlement in issue and tested the evidence relevant to that proposition. Formation analysis must operate at this level of precision. A court should not establish a broad customary rule merely because the evidence establishes a related principle.
Taken together, North Sea and Nicaragua v. Colombia show two complementary aspects of formation. North Sea demonstrates the danger of treating treaty participation and repeated conduct as automatically sufficient to establish custom. Nicaragua demonstrates how sufficiently widespread and uniform practice, assessed in its legal context and over time, can support a finding that the customary requirements have been satisfied. Neither case replaces the underlying evidence; each illustrates how a court reasons from that evidence to the legal conclusion.
The cases also reinforce the importance of explaining the threshold reached. A court should be able to identify why the evidence remains insufficient, why it demonstrates an emerging rule, or why it has become sufficient to recognise an established customary rule. The conclusion should not merely state that a rule is “customary”; it should identify the relevant practice, legal acceptance, period and evidential reasoning that justify the finding.
Evidence is central to the identification of customary international law. A court cannot establish a customary rule merely by asserting that States generally recognise it. The existence, scope, formation and continuing status of a customary rule must be demonstrated through relevant evidence of State practice and opinio juris.
The first task is to identify precisely what proposition the evidence is being used to establish. A proposed rule may contain several elements: a prohibition or permission, the circumstances in which it operates, its personal or geographical scope, exceptions, procedures and consequences. Evidence supporting one proposition does not automatically establish all the others. The Court should formulate the precise customary proposition before weighing the evidence.
Evidence should then be classified according to what it actually demonstrates. A document may show State practice, legal belief, both, neither, or only secondary commentary. The Court should not treat every document containing the words “customary international law” as equal proof of custom.
### Diplomatic Practice and Official Statements
Diplomatic correspondence provides important evidence. States explain positions concerning international incidents through diplomatic notes, protests, responses and formal communications. These materials may demonstrate conduct and may reveal the legal rules a State considers applicable.
Official governmental statements can provide evidence of opinio juris. The Court should consider who made the statement, the authority under which it was made, the circumstances, its precise wording and whether the State maintained the position. A general political declaration that conduct is dangerous or undesirable is not automatically a statement that the conduct is unlawful.
Diplomatic material should be examined chronologically. A State may make an initial claim, qualify it, withdraw an objection, maintain it or change its legal reasoning. The Court should therefore avoid selecting isolated quotations that conceal the development of the State's position.
### Protests, Silence and Acquiescence
Diplomatic protests are particularly useful when responding to concrete conduct. A protest may identify the conduct, identify the alleged international rule and demand cessation or another response. The reaction to the protest may provide further evidence concerning whether the rule is accepted, rejected or disputed.
Silence requires caution. A State may be unaware of conduct, lack a meaningful opportunity to respond, have no sufficient interest or deliberately exercise diplomatic restraint. Absence of protest is therefore not automatically evidence of acceptance.
Silence may become more informative where the State knew of the conduct, possessed a relevant legal interest, had a genuine opportunity to respond and circumstances were such that a response could reasonably have been expected. The Court should explain those circumstances before drawing an inference of acquiescence.
Acquiescence must therefore be distinguished from simple absence of objection. The inference must remain connected to the precise customary proposition being examined.
### National Legislation and Judicial Decisions
Domestic legislation can provide evidence of State practice. It may also provide evidence of opinio juris where the State expressly identifies international law as the reason for adopting the measure.
Legislation cannot automatically be treated as evidence of customary obligation. A statute may implement a treaty, pursue domestic policy or respond to administrative needs. The Court should determine what the legislation actually demonstrates and examine accompanying governmental explanations.
The date and continuing status of legislation matter. Repeal, amendment, official explanations and subsequent practice may affect its evidential significance.
National judicial decisions can demonstrate how a State's legal institutions understand international law. A domestic court may identify a rule as customary, rely upon international practice, reject an asserted customary obligation or distinguish treaty obligations from customary rules.
The authority of the domestic tribunal matters. The Court should consider its constitutional position and whether other State institutions accept or reject the reasoning. A single national judgment does not normally establish a general customary rule by itself, while divergent judgments may demonstrate that a proposed rule or its scope remains unsettled.
### Executive, Administrative and Military Practice
Government and public-authority conduct can demonstrate State practice. Administrative decisions, official policies, instructions and implementation measures may be relevant where they concern the international rule under examination.
Military manuals and official instructions may be particularly important in armed-conflict questions. They can show what a State expects its armed forces to do and whether the conduct is presented as legally required. The Court should determine whether the material reflects treaty obligations, domestic policy, operational guidance or customary international law.
Administrative practice requires similar caution. Repeated conduct may result from domestic procedure or convenience rather than an international legal obligation. The legal explanation accompanying the conduct may therefore be decisive to its evidential significance.
### Treaties, Negotiations and Reservations
Treaties and treaty negotiations can provide important evidence concerning customary international law, but their legal significance must be carefully characterised.
A treaty provision may codify an existing customary rule, contribute to crystallisation or development, or create an obligation binding only upon treaty parties. Widespread participation does not automatically convert every treaty provision into customary international law.
When a State complies with a treaty provision because the treaty binds it, that demonstrates treaty compliance but does not automatically establish independent customary acceptance. Stronger evidence may exist where States expressly state that the provision reflects existing customary law or apply the same rule where the treaty does not govern.
Treaty negotiations may reveal different legal positions. States may describe a provision as codifying existing custom, propose progressive development, reject customary status or accept compromise language without resolving the customary question. These positions must be distinguished.
Reservations can also be evidentially significant. A State may accept a treaty while denying that a provision reflects customary international law, or state that it regards the provision as declaratory of existing custom. Such statements provide evidence of that State's legal position but must be assessed in context.
A treaty may operate simultaneously with customary law. A State can be bound by treaty through consent and independently by custom. A non-party may be bound by custom if the customary requirements are established independently, but treaty obligations cannot simply be extended to a non-party because participation is widespread.
### International Organisations and Resolutions
International organisations provide forums for States to express legal positions. Relevant material includes statements during debates, explanations of votes, formal submissions, proposals, declarations and participation in negotiations.
Resolutions can be significant evidence, but adoption of a resolution does not automatically create customary international law. The Court should examine wording, circumstances of adoption, voting patterns, explanations of votes and subsequent State conduct.
Unanimous adoption may be important, but States may support a political objective without accepting every proposition as legally binding. Majority adoption likewise demonstrates support without automatically establishing general custom. States voting against or abstaining may provide important contrary evidence, particularly where they expressly reject the legal proposition.
### International Judicial Decisions
Article 38 of the Statute of the International Court of Justice identifies international custom as evidence of general practice accepted as law and judicial decisions as subsidiary means for determining rules of law.
Judicial decisions therefore assist in organising and evaluating evidence but do not replace State practice and opinio juris. An earlier judgment may be highly persuasive where it carefully examines practice, legal positions, chronology and contrary evidence, but the Court should consider whether the earlier tribunal addressed the same rule, States, period and evidential record.
A judicial statement that a rule is customary is not itself a substitute for examining the evidence on which the conclusion rests. Judicial methodology is therefore relevant as well as the conclusion reached.
### Scholarly and Expert Material
The writings of highly qualified publicists can assist in identifying and explaining customary international law. Academic works may organise State practice, locate diplomatic material, compare national decisions and trace historical development.
Scholarly writing is not itself State practice or opinio juris. Its value is analytical and supporting. Where possible, the Court should identify the primary evidence underlying an academic conclusion.
Expert reports can organise complex evidence or explain technical matters. An expert's conclusion that a rule is customary does not itself establish the rule. The Court should examine the underlying primary material.
### Reliability, Authenticity and Independence
Evidence must be assessed for reliability. A document may be incomplete, unofficial, translated, paraphrased or derived from a secondary source. Where an important legal conclusion depends upon a particular statement, the Court should, where possible, examine the original or authoritative version and record material uncertainty.
Repetition is not necessarily independent corroboration. Several publications may reproduce one government statement. Counting each publication separately could create an appearance of broad support where there is only one State position. The underlying source should be identified.
The same principle applies where several States use identical language derived from a common treaty, model law or institutional document. Such material remains relevant, but its common source should be recognised. Independently developed practice accompanied by similar legal explanations may provide stronger cumulative evidence.
The Court should consider the source's authority, connection to the State, date, authenticity, context, consistency and proximity to the precise legal question.
### Generality, Consistency and Specially Affected States
Evidence should be assessed for generality, consistency and representativeness. Number alone is insufficient. A smaller body of highly authoritative and consistent evidence may be more informative than a large collection of ambiguous material.
States particularly affected by the proposed rule may have special evidential significance because they possess direct experience and repeated opportunities to act or state a legal position. This does not mean that political power, wealth or prominence automatically gives a State control over customary formation.
Generality must be matched to the proposed rule. Extensive practice from a narrow regional group may support regional custom without establishing a general rule. The Court should not draw a conclusion broader than the community represented by the evidence.
### Contrary Evidence and Change
A proper evidential inquiry must include contrary material. States may reject an alleged rule, behave differently while denying a legal obligation, or object only to its scope or application.
Contrary conduct does not automatically defeat an established customary rule. A State may breach a rule while continuing to recognise it, invoke an exception or dispute only its application. The accompanying legal explanation is therefore essential.
If States consistently condemn contrary conduct as unlawful, their reactions may support the underlying rule. If relevant States defend the conduct as lawful and reject the alleged rule, the evidence may point in the opposite direction.
Customary international law can develop and change. The Court should distinguish genuine changes in practice and opinio juris from isolated departures, temporary circumstances and individual policy decisions. A later change in legal acceptance should not automatically be projected backwards.
The date of evidence is therefore critical. A later statement may confirm an earlier position, demonstrate subsequent development or reflect a change. The Court must determine which possibility the evidence supports.
### Regional, Local and Bilateral Custom
Where regional or local custom is alleged, the Court must identify the relevant community and examine the practice and legal views within that relationship.
Regional custom cannot simply be inferred from evidence outside the relevant region. Local or bilateral custom requires precise evidence concerning the relationship between the States and whether repeated conduct was accompanied by an understanding that it was legally required or permitted.
Historical practice alone does not establish legal custom without evidence concerning its legal character.
### Structured Judicial Method
A structured method prevents unsupported conclusions:
1. Formulate the precise customary rule.
2. Identify the relevant States, subjects, circumstances and period.
3. Collect and classify evidence of State practice.
4. Identify evidence of opinio juris.
5. Assess breadth, consistency, representativeness and specially affected States.
6. Examine contrary practice, protests, silence, objections, exceptions and competing explanations.
7. Assess treaties, negotiations, reservations, institutional material, judicial decisions, scholarly writing and expert evidence according to their proper legal and evidential role.
8. Test reliability, authenticity, authority, independence and chronology.
9. Distinguish an established rule from an emerging rule, political expectation or proposed progressive development.
10. State why the combined evidence does or does not establish the precise rule.
The Court should avoid circular reasoning. It should not assume that repeated conduct proves opinio juris merely because it is repeated, and then use the assumed opinio juris to establish the legal character of the conduct. Equally, a legal statement should not be accepted without considering its context, authority, consistency and relationship to State conduct.
Evidence should be assessed as a whole, but every item does not have equal weight. A specific diplomatic protest by an authorised government representative may have different evidential value from an academic summary. A national statute may demonstrate practice while an accompanying government explanation provides evidence of legal belief. A treaty may establish contractual obligations while negotiations provide evidence concerning customary status.
### Worked Evidential Assessment
Suppose a party claims a customary prohibition on a particular governmental measure. State A issues a formal diplomatic protest stating that the conduct is prohibited by customary international law. State B performs the conduct and replies that no customary prohibition exists. State C has legislation prohibiting the conduct but explains that the legislation implements a treaty. State D has similar legislation but expressly states that the prohibition reflects customary international law. States E and F have repeatedly encountered the conduct but made no formal statement.
The Court should not treat all six States as equal evidence. State A provides direct evidence of opinio juris through its legal protest. State B provides contrary evidence. State C provides evidence of practice, but its treaty explanation limits the extent to which the legislation demonstrates independent customary acceptance. State D provides both practice and an expressed legal position. States E and F provide potentially relevant practice, but their silence cannot automatically be treated as acceptance.
The Court should then examine chronology. If State B consistently rejected the alleged rule from the beginning of the relevant period while States A and D adopted their positions only later, the temporal record differs from one in which all positions existed throughout formation. If State B later accepts the rule while claiming an exception, that change must also be recorded.
The Court should examine reactions. If several States protest State B's conduct as unlawful and State B responds by invoking an exception rather than denying the underlying rule, the evidence may support the prohibition while leaving the exception disputed. If several relevant States support State B and reject the existence of the prohibition, the evidence against general customary formation becomes stronger.
The Court should also test the source of the legislation. If State C's law was adopted solely under a treaty obligation, it should not automatically be counted as independent customary acceptance. If State D applies its law to non-treaty relationships and repeatedly explains that customary international law requires the prohibition, the evidential significance is different.
Finally, the Court should determine whether the complete record establishes the precise rule alleged. It may find a general prohibition without finding the claimed exception, or find an emerging rule without proving that it had formed at the relevant date. The judicial conclusion should match the evidence rather than forcing every item into a simple supporting or opposing category.
This illustrates why formation and evidence are not arithmetic. Breadth, quality, independence, consistency, chronology, legal explanations and contrary material all affect evidential weight.
The Court should also distinguish existence from content. Evidence may establish a general prohibition without establishing a claimed exception, procedure, threshold or remedy. The final judgment should state the customary rule no more broadly than the evidence supports.
If the evidence remains insufficient, the Court should say so. Failure to establish a customary rule is not the same as establishing the opposite rule. Evidential uncertainty should not be converted into a positive prohibition, permission or entitlement merely because one party has failed to prove its case.
### Evidence Management and Judicial Transparency
For the proposed Court, evidence management should be systematic. Important material should be identifiable by State, date, source, type of practice, legal position, geographical relevance and precise proposition supported.
Separate categories should be maintained for State practice, opinio juris, treaty material, institutional material, judicial decisions, academic analysis, expert material and contrary evidence. This classification helps prevent secondary commentary from being mistaken for primary evidence.
A judicial decision should explain the route from evidence to conclusion. It should identify the principal practice relied upon, the material demonstrating legal acceptance, the treatment of contrary evidence and the reasons why the combined record establishes the precise customary rule.
Students should therefore learn to classify a source before relying upon it. They should ask who produced it, when it was produced, what it says, whether it records conduct or legal belief, whether it is primary or secondary evidence, whether its source is authoritative and whether contrary evidence exists.
The central principle is that customary international law is established through evidence. State practice and opinio juris are the foundation. Diplomatic materials, legislation, national judgments, administrative and military practice, treaty materials, international-organisation materials, judicial decisions, scholarly writing and expert analysis assist the Court in identifying and evaluating those elements, but they do not all possess the same legal status or evidential weight.
The final question is not whether a large quantity of international material can be collected. It is whether the material, properly classified and weighed, establishes the precise customary rule at the relevant time and for the relevant subjects. That standard protects the distinction between international expectation, political consensus and legally binding customary international law.
The following section will build upon this evidential methodology and examine the judicial assessment of competing evidence and the difficulties that arise when States disagree about the existence, scope or continuing status of a customary rule.
Where evidence comes from an international organisation, the Court should identify the State positions underlying the institutional material whenever possible. An organisation's report may accurately record State conduct without itself expressing the legal belief of those States. Similarly, a committee's conclusion that a rule is customary is not equivalent to evidence that each participating State accepted that conclusion as law. The Court should therefore distinguish the institution's assessment from the primary State evidence it records.
The same discipline applies to databases and compilations. A collection may be extremely useful for locating national legislation, diplomatic statements or judicial decisions, but the compilation itself is not automatically independent evidence of customary law. The Court should trace important propositions back to their original sources before relying upon them for a legal finding.
### Judicial Application of Evidence
The International Court of Justice's decisions illustrate why evidence must be evaluated rather than counted. In the North Sea Continental Shelf cases, the Court examined whether the equidistance principle had become customary international law independently of the treaty provision in which it appeared. The Court considered the character and extent of State practice and whether States had acted from a sense of legal obligation. The case demonstrates that repeated conduct is not sufficient merely because it is repeated: the Court must determine what the conduct actually demonstrates and whether the customary requirements are satisfied.
The Nicaragua v. United States case provides another important example. The Court examined State practice and opinio juris in determining the customary rules concerning the use of force and non-intervention. It considered practice that was inconsistent with the asserted rule and examined whether such conduct was accompanied by legal acceptance, justification or objection. The significance of the case for evidence is methodological: contrary conduct cannot simply be ignored, but neither does every violation automatically disprove a customary rule. The Court must examine how States themselves characterise the conduct and how other States respond.
These cases also show why judicial decisions must be used carefully. A court may rely upon an earlier judgment as a subsidiary means for determining the law, but the earlier judgment does not become an additional State practice merely because it is authoritative. Where the customary rule remains disputed, the Court should identify the underlying State evidence considered by the earlier tribunal and determine whether that evidence supports the same proposition at the relevant time.
A practical evidence assessment may therefore proceed as follows. Suppose ten States have enacted legislation consistent with an alleged customary rule. Five of those States expressly say that they are implementing a treaty, two describe the legislation as domestic policy, and three state that international law requires the measure. Several other States have protested the alleged rule as unsupported by customary law. The Court should not describe all ten legislative acts as equal evidence of opinio juris. The legislation demonstrates practice, but the accompanying explanations affect what that practice proves. The protests provide contrary evidence of legal belief and must be assessed alongside the supporting material.
If the supporting States nevertheless apply the measure outside their treaty relationships and repeatedly explain that customary international law requires it, that may strengthen the case for independent custom. If the opposing States maintain their rejection and other relevant States remain silent without circumstances supporting an inference of acquiescence, the evidence may remain contested. The proper judicial task is to explain why the combined record does or does not establish the precise rule.
The evidential lesson is therefore that courts reason from sources to propositions. A source is not valuable merely because it is official, numerous or repeated. Its relevance depends upon what it demonstrates, its authority, its independence, its context, its chronology and its relationship to the particular customary rule being alleged.
Customary international law is one of the recognised sources of international law. The central examination issue is whether the evidence establishes both essential elements: State practice and opinio juris.
### 1. The Basic Definition
Customary international law is a rule of international law established through sufficiently relevant State practice accompanied by acceptance of that practice as law.
Remember:
**State practice = what States do.**
**Opinio juris = the belief that the practice is legally required or permitted.**
Both elements must be examined.
### 2. The Examination Method
A strong examination answer should move from the evidence to the legal conclusion rather than beginning with an assumed outcome.
Use this sequence:
1. **Formulate the alleged customary rule precisely.**
2. **Identify the relevant States, circumstances and period.**
3. **Identify State practice.**
4. **Identify evidence of opinio juris.**
5. **Assess generality, consistency and representativeness.**
6. **Consider specially affected States where relevant.**
7. **Examine contrary practice, protests, silence and objections.**
8. **Distinguish treaty obligations from independent customary obligations.**
9. **Use judicial and institutional material according to its proper evidential role.**
10. **Reach a conclusion no broader than the evidence supports.**
The order is important. Students should first identify what the alleged rule actually is and then ask what evidence establishes it.
### 3. Identifying State Practice
State practice may be demonstrated through diplomatic correspondence, official governmental statements, national legislation, executive and administrative conduct, military manuals and instructions, national court decisions, treaty practice, conduct within international organisations, diplomatic protests and other authoritative governmental conduct.
Do not simply list these sources. Explain what the particular source demonstrates.
For example, legislation may demonstrate State conduct, but its reason for adoption may affect whether it also demonstrates opinio juris. A diplomatic protest may strongly demonstrate a legal position, but the underlying conduct must still be identified when assessing State practice.
The examination question is therefore:
**What does this evidence actually prove?**
### 4. Establishing Opinio Juris
Opinio juris concerns the legal character attributed by States to the relevant practice.
Evidence may include statements that a rule is required by international law, diplomatic protests alleging illegality, legal submissions, explanations of votes, national judicial reasoning, official legislative explanations and other authoritative expressions of legal position.
A practice followed merely from convenience, political preference, habit or courtesy does not automatically establish opinio juris.
Equally, a statement that a rule exists does not by itself establish the necessary practice.
Keep the distinction visible:
**Practice is conduct.**
**Opinio juris is legal acceptance.**
**Customary international law requires the two elements to be assessed together.**
### 5. Assessing the Strength of the Practice
Generality concerns whether the practice is sufficiently widespread and representative for the customary rule alleged. Consistency concerns whether the practice is sufficiently coherent to support that rule.
Do not treat custom as a numerical vote. The quality, authority, context and independence of the evidence matter.
Occasional departures do not necessarily destroy a customary rule. A State may breach a rule while continuing to recognise it, invoke an exception or dispute only its application.
Conversely, consistent rejection of the alleged rule is important contrary evidence and must be addressed.
Where specially affected States are relevant, their practice may have particular evidential importance because of their direct involvement in the subject matter. This does not mean that other States become irrelevant or that political power determines customary law.
### 6. Time and Geographical Scope
Always identify the relevant period where the facts make time important.
A rule existing today may not have existed in the same form at an earlier date. Later practice may demonstrate subsequent development without proving that the rule already existed.
The same reasoning applies to geographical scope. The alleged rule may be general, regional, local or bilateral. The evidence must be matched to the community for which the rule is claimed.
### 7. Persistent Objection and Contrary Practice
A State that consistently and clearly objects to an emerging customary rule while it is developing may raise a persistent-objection issue.
Do not state the doctrine mechanically. Ask:
- When was the objection made?
- Was it clear?
- Was it maintained?
- What precise rule was opposed?
- Was the objection made while the rule was forming?
Contrary practice must likewise be analysed rather than ignored. Ask whether the conduct demonstrates rejection of the rule, an asserted exception, a breach accompanied by recognition of the rule, or uncertainty about its content.
The reactions of other States may assist in answering that question. Diplomatic protests can provide evidence of opinio juris. Silence may sometimes have evidential significance, but it is not automatically acceptance.
### 8. Treaties and Custom
Treaty law and customary international law are distinct legal sources.
A treaty may:
- codify an existing customary rule;
- contribute to crystallisation or development of custom; or
- establish obligations operating through the treaty relationship.
Therefore, the existence of a treaty provision does not automatically establish an identical customary rule.
A State complying with a treaty provision may simply be fulfilling its treaty obligation. Look for evidence that States regard the rule as independently binding beyond the treaty relationship.
Equally, a State not party to a treaty may still be bound by an independently established customary rule.
In an examination answer, do not repeat treaty formation mechanics unnecessarily. Identify the separate legal basis relevant to the customary question.
### 9. Judicial and Institutional Material
International judicial decisions can assist in identifying and organising the evidence of customary international law. Article 38 of the Statute of the International Court of Justice is an important reference point.
But a judicial decision does not replace State practice and opinio juris. Where a previous judgment is relied upon, explain, where relevant, the evidential reasoning underlying its conclusion.
International-organisation material must also be treated carefully. A resolution is not automatically customary international law. Consider the wording, circumstances of adoption, State voting positions, explanations of votes and subsequent practice.
Academic writing and expert material can assist in locating and explaining primary evidence, but they do not themselves become State practice or opinio juris.
### 10. Handling Conflicting Evidence
Customary-law examination problems often contain evidence pointing in different directions.
Do not hide the conflict. Identify the strongest evidence on each side and assess its weight.
A useful approach is to classify each source:
- **What did the State do?**
- **What did the State say the law required?**
- **Was the conduct treaty-based or independent?**
- **Was the legal position official and authoritative?**
- **Was the position maintained over time?**
- **Did other States protest, accept or reject it?**
- **Is the evidence independent or merely repeated from another source?**
For example, if several States have legislation supporting an alleged rule, determine why the legislation was adopted. If some legislation implements treaties while other States expressly describe the rule as customary, the evidence does not have one uniform evidential character.
If another State consistently rejects the alleged rule, that contrary position must be addressed. If it accepts the general rule but claims an exception, the existence of the rule and the scope of the exception should be analysed separately.
### 11. Formulate the Rule at the Correct Level
One of the most important examination skills is precision.
If the evidence establishes a broad principle but does not establish a detailed exception, do not claim that the exception is customary.
If the evidence establishes a prohibition but not its precise threshold or procedure, do not silently add those elements.
If the evidence establishes the existence of a customary rule but not its application to the particular facts, keep those questions separate.
Likewise, the existence of a customary obligation does not automatically determine jurisdiction, responsibility, standing or remedy. Those may require separate legal analysis.
### 12. Avoiding Common Errors
Do not say:
- “States do this, therefore it is customary law.”
- “Everyone accepts this, therefore it is law.”
- “There is a treaty, therefore the rule is customary.”
- “A court has said it is customary, therefore the court created the rule.”
Instead, identify the evidence and explain the legal reasoning.
Do not count documents mechanically. Ten publications repeating one government statement do not necessarily constitute ten independent State positions.
Do not treat silence automatically as acceptance.
Do not treat political support as opinio juris.
Do not treat an academic statement as primary evidence.
Do not use a later customary development automatically to prove an earlier rule.
Do not allow a broad principle to silently become a more detailed rule than the evidence supports.
### 13. Judicial Reasoning in an Examination Answer
A strong answer should make the reasoning traceable.
Explain:
- what practice has been identified;
- what evidence demonstrates legal acceptance;
- how widespread and consistent the practice is;
- whether the evidence is representative;
- whether specially affected States matter;
- whether contrary practice exists;
- whether objections have been made;
- whether treaties affect the analysis;
- what judicial or institutional material assists the inquiry; and
- why the combined evidence establishes or fails to establish the precise rule.
Where evidence is divided, acknowledge the uncertainty. If the record does not establish the rule, say so.
Failure to establish a customary rule is not the same as proving that the opposite rule exists.
### 14. A Model Examination Structure
A concise problem answer can use the following structure:
**Issue:** What precise customary rule is alleged?
**Practice:** What States have actually done?
**Generality and consistency:** Is the practice sufficiently widespread, representative and coherent?
**Opinio juris:** What evidence shows that States regard the practice as legally required or permitted?
**Contrary evidence:** Which States reject the rule, and why?
**Time:** Did the rule exist at the relevant date?
**Scope:** Is the claim general, regional, local or bilateral?
**Treaty relationship:** Is the obligation treaty-based, customary, or potentially both?
**Authority:** What do relevant judicial and institutional materials contribute?
**Conclusion:** Is the precise customary rule established on the evidence?
This structure should be adapted to the facts. Not every issue will require equal treatment.
### 15. Final Examination Formula
For examination purposes, remember:
**CUSTOM = STATE PRACTICE + OPINIO JURIS**
Then test:
**GENERALITY + CONSISTENCY + REPRESENTATIVENESS + LEGAL ACCEPTANCE**
Then ask:
**TIME + GEOGRAPHICAL SCOPE + CONTRARY PRACTICE + OBJECTIONS**
Finally ask:
**Is the obligation treaty-based, customary, based on another recognised source, or supported by more than one legal basis?**
The examination objective is not merely to reproduce the definition of custom. It is to demonstrate the ability to move from evidence to legal analysis.
A candidate should be able to identify relevant practice, evaluate legal acceptance, distinguish competing sources, recognise uncertainty, address contrary evidence and formulate a conclusion proportionate to the evidence.
The strongest answer will therefore do more than state that customary international law requires State practice and opinio juris. It will demonstrate how those elements operate in the particular factual circumstances.
For judicial training within the proposed Court, the same discipline applies. A future judgment should allow a reader to see why particular evidence was accepted, why other evidence was given limited weight and how the accepted evidence supports the legal conclusion.
The final principle is simple:
**Do not start with the answer and search for supporting evidence. Start with the evidence, identify what it establishes, apply the customary-law requirements, and then state the conclusion.**
The detailed substantive teaching and authorities appear in Sections 1–6. This section is the examination framework for applying that knowledge efficiently and accurately.
### 16. Worked Examination Problem
Consider a problem in which State A claims that a particular governmental action is prohibited by customary international law. The facts state that twelve States have adopted legislation restricting the action, six have issued diplomatic statements describing the action as unlawful, four States continue to permit it and several States have taken no public position. A treaty containing a similar prohibition has twenty parties, but State A and several of the States involved are not parties.
A weak answer would count the twelve laws and six statements and conclude that the rule is customary. A stronger answer asks what each item establishes. The legislation is evidence of State practice, but the reasons for adopting it must be examined. The six statements may provide evidence of opinio juris, particularly if they expressly identify the prohibition as a rule of international law. The four States permitting the conduct provide contrary evidence, and their legal explanations must be considered. The treaty may provide evidence relevant to the development or codification of the rule, but treaty participation does not by itself establish an identical customary obligation.
The answer should then consider whether the practice is sufficiently general and representative, whether the relevant period is important, whether the opposing States are particularly affected, and whether the evidence demonstrates an established rule or merely an emerging one.
The conclusion should be qualified according to the evidence. If the evidence establishes the general prohibition but not a claimed exception, the answer should recognise the prohibition while treating the exception separately. If the evidence does not establish sufficient opinio juris, the correct conclusion may be that customary status has not been demonstrated. This is a stronger legal answer than forcing the facts into a “yes” or “no” conclusion unsupported by the evidence.
### 17. Applying Judicial Authority in an Examination
Cases should be used for reasoning, not decoration. If the facts raise the formation of custom, a student may use the reasoning in the North Sea Continental Shelf cases to explain why repeated conduct or treaty language cannot simply be counted as proof of customary law. If the problem concerns inconsistent State conduct, the reasoning in Nicaragua v. United States can assist in explaining why contrary conduct must be examined together with the legal positions accompanying it.
The case should then be connected to the facts. A statement such as “North Sea says practice must be widespread” is less useful than explaining how the evidence in the problem compares with the evidential problem addressed by the Court. The examiner should be able to see that the candidate understands the principle and can apply it.
Judicial decisions should also be used proportionately. A short examination answer does not require a long history of every authority. One well-used authority, accurately connected to the issue, is normally more valuable than several cases cited without explanation.
### 18. Model Analytical Paragraph
A useful model paragraph is:
“The evidence does not permit the Court simply to infer customary law from repeated conduct. The legislation demonstrates State practice, but several States adopted their measures pursuant to treaty obligations, which limits its value as independent evidence of custom. The diplomatic statements of States expressly describing the prohibition as required by international law provide stronger evidence of opinio juris. The States continuing to permit the conduct provide contrary evidence that must be addressed. The Court must therefore determine whether the supporting practice is sufficiently general and representative and whether the accompanying legal positions demonstrate acceptance as law at the relevant date. On the present facts, the evidence may support an emerging rule, but the customary prohibition can be recognised only to the extent established by the combined record.”
The value of this structure is that every conclusion is connected to evidence. It avoids unsupported assertions while remaining concise.
### 19. Recognising the Strongest and Weakest Evidence
In an examination, students should prioritise evidence rather than treating every fact as equally important.
An express official statement that a State considers a rule legally binding may be highly relevant to opinio juris. Actual governmental conduct may be important to State practice. A treaty provision may be significant but requires analysis of its relationship with custom. A resolution may record important State positions but does not automatically become customary law. An academic article may identify the authorities but normally provides supporting analysis rather than primary proof.
Where time is limited, students should identify the evidence most directly connected to the alleged rule and explain its significance. A long list of less relevant sources can obscure the decisive material.
### 20. Dealing With Uncertainty
International-law examination questions may deliberately provide an incomplete evidential record. The correct response is not to invent missing evidence.
If practice is widespread but opinio juris is unclear, say that the practice element appears stronger than the legal-acceptance element.
If opinio juris is expressed clearly but actual State practice is limited, identify the weakness in practice.
If both elements appear substantial but important States consistently object, address the contrary evidence before reaching a conclusion.
If the evidence supports several interpretations, explain which interpretation is strongest and why. If the evidence remains insufficient, state that the customary rule has not been established on the facts provided.
This demonstrates legal judgment. An answer does not become stronger by expressing certainty that the evidence cannot support.
### 21. Distinguishing the Legal Question From the Factual Question
Students should separate factual findings from legal conclusions.
“The State enacted legislation in 2018” is a factual proposition.
“The legislation demonstrates State practice” is an evidential assessment.
“The State regarded the practice as legally required” is a conclusion about opinio juris.
“The proposed prohibition is customary international law” is the final legal conclusion.
Keeping these stages separate prevents reasoning from becoming circular. The examiner should be able to follow the movement from fact, to evidence, to legal element, to rule.
### 22. Examination Under Time Pressure
Where examination time is limited, the full methodology can be compressed without losing legal accuracy.
Begin with the alleged rule.
Then identify the strongest evidence of practice and the strongest evidence of opinio juris.
Immediately identify important contrary evidence.
Check whether the facts raise time, geographical scope, specially affected States, persistent objection or a treaty relationship.
Use one or two relevant authorities to support the reasoning.
Finish with a precise conclusion.
A concise answer can therefore remain legally sophisticated. The objective is not to reproduce the whole chapter but to demonstrate that the candidate knows which questions must be asked and can answer them from the facts.
### 23. What High-Quality Answers Demonstrate
A high-quality examination answer demonstrates five things.
First, **precision**: the candidate identifies the exact customary rule being claimed.
Second, **classification**: the candidate distinguishes State practice, opinio juris, treaty evidence, judicial authority and secondary material.
Third, **evaluation**: the candidate weighs supporting and contrary evidence rather than merely listing it.
Fourth, **application**: the candidate connects the legal principles to the facts.
Fifth, **proportionate conclusion**: the candidate states no more than the evidence establishes.
An answer can therefore be relatively short and still demonstrate substantial knowledge if every important proposition is supported by appropriate reasoning.
### 24. Examination Checklist
Before submitting an answer, ask:
- Have I identified the exact customary rule?
- Have I identified actual State practice?
- Have I separately addressed opinio juris?
- Have I assessed generality and consistency?
- Have I considered representativeness and specially affected States where relevant?
- Have I checked the relevant date?
- Have I considered regional, local or bilateral scope where appropriate?
- Have I addressed contrary practice?
- Have I considered persistent objection if the facts raise it?
- Have I distinguished treaty obligations from custom?
- Have I used judicial authority for reasoning rather than as a substitute for evidence?
- Have I avoided treating silence as automatic acceptance?
- Have I avoided treating political support as opinio juris?
- Have I separated existence of a rule from its application and consequences?
- Does my conclusion match the strength of the evidence?
If the answer to these questions is yes, the candidate has applied the chapter's methodology rather than merely memorised its terminology.
### 25. Final Examination Principle
The purpose of this section is to convert the substantive teaching in Sections 1–6 into examination performance.
A candidate should not attempt to reproduce every authority or every category of evidence. The candidate should identify the decisive legal issue, select the relevant evidence, explain what that evidence demonstrates, address the strongest counterargument and reach a measured conclusion.
The examiner should therefore be looking for a chain of reasoning:
**FACTS → EVIDENCE → STATE PRACTICE / OPINIO JURIS → CUSTOMARY RULE → APPLICATION → CONCLUSION**
If a link in that chain is missing, the answer may contain correct terminology but still fail to demonstrate legal reasoning.
The strongest examination response is consequently not the longest response. It is the response that uses the available facts to demonstrate why a customary rule is established, disputed, emerging or insufficiently proved.
The detailed substantive teaching remains in Sections 1–6. Section 7 provides the practical examination framework through which that knowledge is demonstrated under assessment conditions. Section 8 will provide the final learning and application component of the chapter.
Customary international law is a recognised source of international law.
The central rule to remember is:
**CUSTOMARY INTERNATIONAL LAW = STATE PRACTICE + OPINIO JURIS**
State practice concerns the conduct of States.
Opinio juris concerns the acceptance of that conduct as legally required or permitted.
Both elements must be considered together.
### Core Principles
State practice may be found in:
- diplomatic correspondence;
- official government statements;
- national legislation;
- executive and administrative practice;
- military manuals and instructions;
- national court decisions;
- treaty practice;
- conduct within international organisations; and
- diplomatic protests and responses.
Opinio juris may be demonstrated through:
- statements that conduct is legally required;
- diplomatic objections based on international law;
- legal submissions;
- explanations of votes;
- national judicial reasoning;
- official legislative explanations; and
- other authoritative governmental statements.
### Assessing Custom
When deciding whether a customary rule exists, consider:
1. **Generality** — is the practice sufficiently widespread?
2. **Consistency** — is the practice sufficiently consistent?
3. **Representativeness** — does the evidence properly reflect the international community and, where relevant, specially affected States?
4. **Legal acceptance** — is there evidence of opinio juris?
5. **Contrary practice** — are there significant objections or conflicting conduct?
6. **Context** — why are States behaving in the way they do?
Do not assume that repetition alone creates law.
### Treaties and Custom
Treaty law and customary international law are distinct legal sources.
A treaty may:
- codify an existing customary rule;
- contribute to the development or crystallisation of custom; or
- create treaty obligations that operate independently between its parties.
A treaty provision therefore does not automatically become customary international law.
Equally, a State's non-participation in a treaty does not automatically mean that no customary rule applies to it.
The detailed treaty mechanics have already been developed in earlier chapters. For customary-law questions, the important task is to determine whether the evidence establishes an **independent customary legal basis**.
### Evidence
Courts may consider:
- State practice;
- official statements;
- diplomatic materials;
- legislation;
- national judgments;
- military manuals;
- treaty practice;
- international organisation materials;
- international judicial decisions; and
- scholarly writing.
International judicial decisions assist in identifying and explaining customary rules, but they do not replace State practice and opinio juris.
Article 38 of the Statute of the International Court of Justice is a key reference because it describes international custom as evidence of a general practice accepted as law.
### Persistent Objection
A State that consistently and clearly objects to an emerging customary rule may raise the persistent-objection doctrine.
The timing and consistency of the objection are important. A later disagreement with an established rule should not automatically be treated as a persistent objection during formation.
### Judicial Method
A court examining an alleged customary rule should:
1. identify the precise rule;
2. identify relevant State practice;
3. assess its generality and consistency;
4. identify evidence of opinio juris;
5. consider contrary practice and objections;
6. consider specially affected States where relevant;
7. distinguish treaty obligations from customary obligations;
8. consider relevant judicial and institutional material; and
9. explain why the evidence establishes or fails to establish the rule.
### Final Examination Formula
Remember:
**What do States do?** → State practice.
**Why do they do it in legal terms?** → Opinio juris.
**Is the practice sufficiently general and consistent?** → Assess the evidence.
**Is there contrary practice or objection?** → Address it.
**Is there a treaty involved?** → Identify whether the obligation is treaty-based, customary, or supported by both.
**What does the evidence establish?** → Give a reasoned legal conclusion.
The detailed teaching appears in Sections 1–6. Section 7 provides the examination framework. This final revision section is intended to consolidate the framework for revision rather than reproduce the substantive discussion.
The essential principle remains:
**Customary international law is identified through sufficiently relevant State practice accompanied by acceptance of that practice as law.**
A careful court must demonstrate the evidence supporting both elements before declaring a customary rule established.
Customary international law should be revised as an evidence-based source of international legal obligation. The essential starting point is that customary international law is identified through State practice accompanied by opinio juris. These two elements are related but distinct and must not be collapsed into a single inquiry.
State practice concerns what States do. Relevant material may include diplomatic correspondence, official statements, legislation, administrative conduct, military manuals, national judicial decisions, treaty practice and conduct within international organisations. The relevance and weight of each item depend upon its source, context and connection to the rule being examined.
Opinio juris concerns the legal character attributed to the practice. The Court asks whether States regard the relevant conduct as legally required or permitted. Conduct arising only from convenience, political preference, economic interest, reciprocity, habit or moral preference does not automatically establish opinio juris.
The two elements must be assessed together. Extensive practice without evidence of legal acceptance may demonstrate habit rather than law. Conversely, statements asserting that a rule exists do not necessarily establish the rule without sufficiently relevant practice.
The alleged rule should always be formulated precisely. Evidence supporting a broad principle does not automatically establish every detailed obligation, exception, threshold or procedural consequence that a party may seek to derive from it.
Generality, consistency and representativeness are important considerations. A general customary rule requires sufficiently broad and representative evidence. Specially affected States may have particular evidential importance in appropriate fields, while regional or local customs require attention to the relevant group of States.
Time also matters. A customary rule may develop or change. Evidence from a later period should not automatically be projected backwards to establish that an identical rule existed earlier. The Court should identify the relevant period whenever the facts make timing material.
Contrary State practice must be considered. Conduct inconsistent with an alleged rule may represent rejection, an exception, a breach accompanied by continued recognition of the rule, or uncertainty about the rule's content. The reactions and legal explanations of States may assist in determining its significance.
Persistent objection may become relevant where a State consistently and clearly objects to an emerging customary rule. The timing and consistency of the objection must be examined rather than treating the doctrine as automatic.
Treaties must also be distinguished from custom. A treaty may codify an existing customary rule, contribute to its crystallisation, influence later development, or establish obligations that remain treaty-based. Treaty participation or compliance alone does not prove that an identical customary rule exists independently.
International judicial decisions can assist in identifying and explaining custom, but courts do not simply create customary international law through judgments. Judicial reasoning should be connected to the underlying evidence of State practice and opinio juris.
The principal examination method is therefore: identify the precise alleged rule; identify the relevant period and States; collect State practice; identify opinio juris; assess generality, consistency and representativeness; consider contrary evidence and persistent objection where relevant; distinguish treaty obligations; and then determine whether the evidence establishes the precise rule claimed.
Finally, distinguish identification of the rule from application of the rule. Even where customary status is established, separate questions concerning jurisdiction, attribution, breach, exceptions, responsibility and remedy may remain.
The central revision formula is:
STATE PRACTICE = what States do.
OPINIO JURIS = the belief that the relevant practice is legally required or permitted.
CUSTOMARY INTERNATIONAL LAW = the legally significant combination of both elements.
The strongest examination answer does not merely state this formula. It applies it to the evidence, addresses competing material and reaches a conclusion proportionate to what the evidence actually establishes.
For revision purposes, the evidence used to identify custom should be organised according to the proposition it supports. Evidence of physical or governmental conduct primarily assists with the practice element. Evidence showing that States regard conduct as legally required or permitted assists with opinio juris. Some material may be relevant to both, but the Court should explain the particular evidential function being relied upon.
Official statements are important because they can reveal a State's legal position. Diplomatic protests may demonstrate that a State considers particular conduct unlawful. National legislation and administrative measures may demonstrate governmental practice, while national court decisions may reveal the domestic legal understanding of international rules. Military manuals can be particularly relevant where the alleged custom concerns armed conflict.
International organisation practice should be assessed carefully. Statements made by States within an organisation may provide evidence of their legal positions. A resolution itself should not automatically be treated as a customary rule. Its wording, voting circumstances, explanations of votes, subsequent conduct and the positions of participating States may all affect its evidential value.
Judicial decisions are valuable for identifying and explaining customary international law. They may collect evidence, compare State positions and formulate the methodology used to determine whether a rule exists. Their conclusions should nevertheless be understood in connection with the underlying evidence rather than treated as independent proof that a rule exists.
The quality of evidence matters as much as its quantity. Ten ambiguous examples do not necessarily provide stronger evidence than several clear and authoritative examples. The Court should consider whether the evidence is official, attributable, relevant, consistent, representative and sufficiently connected to the precise rule asserted.
Contrary evidence is especially important. If several States act inconsistently with the proposed rule, the Court should determine whether those acts represent rejection of the rule, claimed exceptions, breaches, changes in legal position or uncertainty. Ignoring contrary practice can make a customary-law conclusion appear stronger than the evidence permits.
The Court should also consider reactions to conduct. Protests may support the existence of opinio juris, particularly where they expressly invoke international law. Silence must be approached cautiously because failure to protest may have many explanations. Its significance depends upon awareness, opportunity to respond and the circumstances in which a response might reasonably have been expected.
A further revision point is the difference between a general principle and a precise customary rule. Evidence may establish that States recognise a broad principle without establishing every detailed consequence proposed by a party. The Court should formulate the alleged rule carefully and test each significant component against the evidence.
Customary international law can also develop over time. A rule may emerge through changing practice and legal acceptance. Later evidence may demonstrate development without proving that the same rule existed at an earlier date. Students should therefore pay attention to the temporal question whenever the facts specify a historical period.
Geographical scope should likewise be considered. A general customary rule requires broad evidence, while regional or local custom concerns a more limited group of States. The facts should determine which form of custom is being alleged.
Persistent objection may be relevant where a State consistently and clearly objects to an emerging customary rule. The timing, clarity and continuity of the objection must be examined. The doctrine should not be invoked merely because a State once expressed disagreement.
Treaty law and customary law may overlap but remain distinct sources. A treaty can codify existing custom, contribute to crystallisation, encourage later development or establish obligations applicable only through the treaty. The Court must identify which legal basis supports the conclusion.
The final revision sequence is therefore:
1. Identify the exact alleged customary rule.
2. Identify the relevant time period.
3. Identify the relevant States and geographical scope.
4. Collect evidence of State practice.
5. Collect evidence of opinio juris.
6. Assess generality, consistency and representativeness.
7. Consider specially affected States where relevant.
8. Examine contrary practice and reactions.
9. Consider persistent objection where relevant.
10. Distinguish treaties and other sources of international law.
11. Assess judicial and academic authorities according to their proper roles.
12. Decide whether the precise rule has been established.
13. Apply that rule separately to the facts.
14. Address jurisdiction, responsibility, exceptions and remedies separately where required.
The key distinction to remember is that customary international law is not simply what States commonly do. It is what the evidence demonstrates that States practise in circumstances accompanied by acceptance of the practice as law. The Court's task is to identify that legal character through disciplined examination of the available evidence.
A good revision answer should therefore move from definition to evidence, from evidence to legal analysis, and from legal analysis to application. It should distinguish certainty from uncertainty, established custom from emerging custom, general custom from regional or local custom, and customary obligations from treaty obligations.
For the proposed Court, this methodology should form the foundation of both examination and judicial reasoning. A participant who can apply this sequence to unfamiliar facts demonstrates a deeper understanding than one who can merely reproduce the terminology. The objective is ultimately to ensure that findings of customary international law are evidence-based, transparent and capable of being tested by the parties.
When revising the chapter, students should remember that the Court is not required to find identical conduct in every State before customary international law can exist. The relevant inquiry concerns sufficiently general and representative practice assessed in light of the subject matter. Differences in implementation do not necessarily defeat a rule where the underlying legal position remains sufficiently consistent.
At the same time, inconsistency cannot simply be ignored. Where States repeatedly take contradictory positions, the Court must determine whether the differences concern the existence of the rule, its scope, exceptions or the factual circumstances in which it applies. A conclusion that custom exists should address significant contradictions rather than selecting only favourable examples.
The practice of specially affected States may have particular significance where the subject matter is closely connected to a limited group of States. Their conduct may provide important evidence because those States have greater practical involvement with the activity. However, their practice should still be assessed alongside the wider international record.
The passage of time is not governed by a simple numerical requirement. Some customary rules may develop over long periods, while rapid developments may occur where practice and legal acceptance become sufficiently clear within a shorter period. The question is always whether the evidence demonstrates the necessary elements of custom, not whether an arbitrary number of years has passed.
The distinction between emerging and established custom is therefore important. An emerging rule may have substantial support without yet satisfying the requirements for a customary rule. Students should distinguish evidence showing movement towards a rule from evidence establishing that the rule has already become binding international law.
The same distinction applies to proposed changes in customary law. A rule may evolve where State practice and opinio juris change sufficiently over time. Political change alone is not enough. The Court should identify evidence showing that the legal position of States has actually developed.
Persistent objection should be approached within this evidential framework. The relevant question is not simply whether a State dislikes the proposed rule. The Court should examine whether the State objected clearly, consistently and at the relevant stage of the rule's development. The significance of the objection must then be considered in relation to the customary rule alleged.
The regional and local dimensions of custom should also be kept separate from general customary international law. Where evidence concerns a particular group of States, the Court should determine whether the material establishes a regional or local rule rather than automatically treating it as a general rule binding across the international community.
Another important revision point is that State practice can include conduct that appears inconsistent with an alleged rule. A breach does not necessarily demonstrate that the rule has ceased to exist. If the State acknowledges that its conduct is unlawful while nevertheless acting contrary to the rule, the conduct may actually support recognition of the rule. The Court must examine the State's explanation and the reactions of other States.
Evidence should also be evaluated chronologically. Earlier evidence may establish the origin or historical development of a rule, while later evidence may demonstrate consolidation, modification or rejection. A judgment should make clear which period of evidence supports the conclusion being reached.
The Court should distinguish primary evidence from secondary analysis. State documents, official statements, diplomatic correspondence and governmental conduct are primary evidence concerning State practice and legal position. Judicial decisions and academic writing may analyse that evidence and assist the Court, but they should not be confused with the conduct of States themselves.
Where a previous international judgment contains an extensive analysis of custom, the Court should consider both its authority and its evidential reasoning. A later court may rely upon the methodology while still examining whether the underlying State practice and opinio juris support applying the rule to the new dispute.
Examination questions may also test whether students understand the difference between legal obligation and political commitment. A State may support a declaration, resolution or policy for political or humanitarian reasons without accepting that the relevant conduct is legally required. The Court must identify evidence of legal acceptance rather than assuming that political support equals opinio juris.
Likewise, moral approval does not automatically create customary law. A principle may be widely regarded as desirable while the evidence remains insufficient to establish a customary obligation. Conversely, an established customary rule remains legally relevant even if a State considers its policy consequences undesirable.
The strongest revision approach is therefore to treat every fact in a problem as potentially evidential. A diplomatic protest may concern opinio juris. A repeated governmental action may concern State practice. A treaty provision may raise questions of codification or crystallisation. A contradictory State statement may affect the weight of the evidence. A judicial decision may provide an analytical framework. The student should identify the legal significance of each fact rather than merely repeating it.
The final conclusion should answer the actual legal question. If the question asks whether a rule exists, determine whether practice and opinio juris establish it. If it asks when the rule emerged, address the temporal evidence. If it asks whether a particular State is bound, consider the relevant customary rule and any applicable doctrines such as persistent objection. If it asks whether conduct violates the rule, first establish the rule and then separately analyse the facts.
A complete revision of this chapter should therefore leave the student able to explain not only the definition of customary international law but also the practical method by which a court identifies it. The student should understand the evidence, the competing interpretations, the limits of judicial authority and the importance of transparent reasoning.
The ultimate purpose of the chapter is to ensure that customary international law is treated as a disciplined legal source. The Court should neither assume custom from popularity nor reject it merely because no treaty contains the relevant rule. It should identify the evidence, evaluate State practice and opinio juris, formulate the rule precisely and explain the conclusion reached.
The proposed Court should apply the same methodology consistently across cases. This will help ensure that findings concerning customary international law are predictable, transparent and capable of review. The methodology itself should remain stable even where the substantive subject matter changes.
In final revision, remember the central chain:
Evidence of conduct → State practice.
Evidence of legal acceptance → opinio juris.
Sufficiently general, consistent and representative evidence → possible customary rule.
Precise legal formulation → defined content and scope.
Application to the facts → determination of whether the rule governs the dispute.
Separate analysis → jurisdiction, responsibility, exceptions and remedies.
This chain captures the essential reasoning structure of the chapter. A candidate who can apply it to unfamiliar facts demonstrates substantive understanding rather than simple recall.
A final revision point is the importance of distinguishing legal evidence from assertion. A party may state that a rule is customary, but the assertion must be tested against the international record. The Court should ask what States have actually done, what States have said about the legal significance of that conduct and whether the evidence supports the precise rule proposed.
Students should also remember that evidence may have different degrees of weight. An official diplomatic protest expressly invoking international law may be particularly useful evidence of legal position. An isolated unofficial statement may have much less weight. A national judgment may demonstrate the position of one State's courts without establishing a general international rule. A resolution may record important State positions without itself constituting custom.
Where evidence points in different directions, the Court should explain the conflict. It should not conceal uncertainty by describing disputed propositions as settled law. A carefully qualified conclusion can be legally stronger than an absolute conclusion unsupported by the record.
The examination should also distinguish the existence of a customary rule from its content, scope and application. The Court may establish a general rule but still need to determine whether an exception exists, whether the rule applied during the relevant period, whether the particular State is affected by a recognised qualification, and whether the facts fall within the rule.
The complete revision method can therefore be remembered as an evidential sequence:
Identify the alleged rule.
Identify the relevant States, period and geographical scope.
Collect State practice.
Identify opinio juris.
Assess generality, consistency and representativeness.
Examine specially affected States where relevant.
Consider contrary practice, protests, silence and possible breaches.
Consider persistent objection where applicable.
Distinguish treaty obligations from customary obligations.
Assess judicial decisions and academic commentary according to their proper roles.
Determine whether the precise rule has been established.
Apply the rule separately to the facts.
Address jurisdiction, responsibility, exceptions and remedies separately.
The central lesson of Chapter 7 is therefore not simply that custom requires State practice and opinio juris. The deeper lesson is that a court must demonstrate the evidential path by which those elements establish a particular rule. Customary international law is identified through disciplined legal reasoning rather than assumption.
For examination purposes, a candidate should be able to move from a factual record to a legally reasoned conclusion. The candidate should identify relevant evidence, classify its significance, address contrary material, distinguish competing sources and formulate a conclusion proportionate to the evidence. That is the standard expected of a participant being prepared for serious international judicial work.
For the proposed Court, the same methodology provides a foundation for transparent judgments. A decision concerning customary international law should allow the parties to understand what evidence was relied upon, what competing evidence was considered and why the Court concluded that the requirements for custom were or were not satisfied.
The final formula is:
STATE PRACTICE — what States do.
OPINIO JURIS — the acceptance of the practice as legally required or permitted.
CUSTOMARY RULE — the precise legal proposition established by the combined evidence.
APPLICATION — whether that rule governs the facts before the Court.
CONSEQUENCES — any separate questions concerning jurisdiction, responsibility, exceptions or remedies.
This framework completes the revision of customary international law. Students should be able to use it without relying upon memorised wording, applying the methodology to new facts and unfamiliar disputes. The objective is understanding, evidence-based reasoning and accurate legal conclusion.
FINAL CHAPTER ASSESSMENT
Test Your Knowledge
This final assessment covers Sections 1–9 and tests deeper understanding, application and legal reasoning.
1. A court must determine whether a claimed rule has become customary international law. Which inquiry best reflects the legal test?
2. A State repeatedly follows a particular practice but explains that it acts this way for political convenience rather than legal obligation. What is the strongest legal assessment?
3. A proposed customary rule is supported by extensive practice, but important States consistently act differently. What should the court assess before reaching a conclusion?
4. A State formally protests another State's conduct and states that the conduct is contrary to international law. What evidential significance may the protest have?
5. A national court applies an international legal rule in deciding a domestic case. How may the decision assist an inquiry into customary international law?
6. A treaty contains a provision resembling a claimed customary rule, but a State involved in the dispute is not a party to the treaty. What must the court determine?
7. A customary rule appears to be developing, with increasing supporting practice but continuing disagreement among States about its legal character. Which conclusion is most appropriate?
8. A State objects clearly and consistently to an emerging customary rule while the rule is developing. Which issue should the court consider?
9. An international organisation adopts a resolution supporting a proposed customary rule, but State practice remains mixed. How should the resolution be treated?
10. A court accepts that a customary rule exists but must decide whether it governs the conduct in dispute. What should the court do next?
11. A claimed customary rule developed rapidly over several years. Which factor is most important when assessing whether the rule has already formed?
12. Several States depart from an established customary rule but continue to describe their conduct as an exceptional breach. What may this indicate?
13. A customary rule concerns an activity undertaken primarily by a limited group of States. Why may those States receive particular attention?
14. A State begins objecting to a customary rule only after the rule appears to have become established. What issue should the court distinguish?
15. A customary rule is alleged to apply only within a particular geographical region. What must a court establish before recognising the claimed regional rule?
16. During negotiations within an international organisation, States repeatedly explain that a proposed standard reflects international legal obligations. What can this material contribute?
17. An international court refers to earlier judgments when analysing a customary rule, but the underlying State practice has changed since those judgments. What should a later court do?
18. A court consults several scholarly works that disagree about whether a customary rule exists. What is the most appropriate use of those writings?
19. A proposed customary rule is supported by numerous examples of conduct, but several States expressly reject the rule on legal grounds. What should a court do?
20. A court is asked to apply an alleged customary rule to a dispute involving conduct not previously considered by the parties. What is the strongest judicial approach?
21. A State introduces legislation reflecting a proposed customary rule and later defends the legislation in diplomatic statements using legal reasoning. How should this material be assessed?
22. States generally follow a particular practice, but some departures occur and the States concerned describe those departures as exceptional. What may the explanations indicate?
23. A government follows a practice because officials believe it is efficient and politically advantageous, while making no claim that international law requires the conduct. What does this most directly illustrate?
24. A court finds extensive practice supporting a proposed customary rule but also identifies substantial contrary conduct. Which approach is most consistent with an evidence-based determination?
25. A treaty codifies a rule that States may also have followed before the treaty was adopted. How should a court approach the customary-law question?
26. A judicial decision identifies a customary rule but gives limited reasons for its conclusion and relies heavily on earlier decisions. What should a later court consider?
27. A court finds that several textbooks describe a customary rule but discovers that the authors rely on the same small group of State statements. What should the court recognise?
28. A proposed rule is supported by conduct from many States and several legal statements, but the evidence concerns a broad principle rather than the precise rule claimed in court. What should follow?
29. A State consistently protests conduct as unlawful but occasionally tolerates similar conduct by another State for diplomatic reasons. How should the evidence be approached?
30. A court concludes that the evidence establishes a customary rule prohibiting particular conduct. The respondent argues that its conduct falls outside the rule's scope. What must the court determine?
31. A State argues that a customary rule exists because many governments have followed the same practice for decades, but the evidence gives different reasons for that conduct. What should the court examine most carefully?
32. An international organisation adopts two resolutions concerning a proposed customary rule, but the wording of the second resolution is significantly less supportive than the first. What should a court examine?
33. A State consistently opposed a proposed customary rule during its development but later stopped making public objections while continuing to avoid the relevant conduct. What should a court examine?
34. A customary rule concerns a specialised maritime activity and most evidence comes from States that regularly conduct that activity. What should the court assess?
35. A treaty provision resembles a customary rule, but States have interpreted the treaty provision differently from the alleged customary rule. What should a court do?
36. A State repeatedly performs an act and publicly describes it as a matter of national policy rather than international legal obligation. What should a court infer?
37. A national supreme court states that international law requires a particular conduct, but the executive branch has publicly described the same conduct as voluntary policy. How should the evidence be assessed?
38. Evidence supporting a customary rule is substantial, but contrary practice is concentrated among several States that expressly reject the alleged legal obligation. What should the court determine?
39. A court determines that customary law prohibits a particular form of conduct, but the evidence does not establish a claimed procedural requirement associated with that prohibition. What is the proper judicial conclusion?
40. A court is asked to decide whether a disputed customary rule applies to the facts of a case. Which conclusion best reflects the complete F5 methodology?
Customary International Law
International law arising from sufficiently established State practice accepted as law.
State Practice
Conduct attributable to States that may contribute to the formation or identification of customary international law.
Opinio Juris
The acceptance or belief that particular conduct is legally required, permitted or recognised by international law.
General Practice
Practice sufficiently widespread and relevant to support identification of a customary rule.
Consistency
The degree to which relevant State conduct follows a sufficiently coherent pattern.
Generality
The breadth of participation in the relevant international practice.
Representativeness
The extent to which the evidence reflects the relevant international community.
Specially Affected States
States whose direct involvement in the subject matter may give their practice particular evidential significance.
Persistent Objection
Consistent and clear objection by a State to an emerging customary rule during its formation.
Diplomatic Protest
An official objection by a State asserting that another State's conduct is inconsistent with international law.
National Legislation
Domestic legislation that may provide evidence of State practice or a State's legal position.
National Judicial Decisions
Domestic court decisions that may provide evidence concerning a State's understanding of international law.
Treaty Practice
State conduct and positions relating to treaties that may provide evidence relevant to customary international law.
Codification
The expression in treaty form of rules that may already exist as customary international law.
Crystallisation
The development of a customary rule through interaction between emerging practice and legal acceptance.
International Organisation
An international institution within which States may express positions relevant to international law.
Judicial Decision
A decision of a court or tribunal that may assist in identifying and interpreting international legal rules.
Scholarly Writing
Academic analysis that may assist courts in organising and evaluating evidence of international law.
Contrary Practice
State conduct that conflicts with an alleged customary rule and must be considered in assessing the evidence.
Legal Acceptance
Recognition by States that particular conduct has legal significance under international law.
International Legal Obligation
A requirement arising from a recognised source of international law.
Practice Accepted as Law
The combination of relevant State conduct and acceptance of that conduct as legally significant.
Evidence of Custom
Material used by a court to assess State practice and opinio juris when identifying customary international law.
Article 38
Article 38 of the Statute of the International Court of Justice, including its reference to international custom.
International Court of Justice
The principal judicial organ of the United Nations whose decisions and advisory opinions may assist in identifying, interpreting and applying rules of international law, including customary international law.
Regional Custom
A customary rule applying within a particular region where the relevant States' practice and legal acceptance establish it.
State Conduct
Acts or omissions attributable to a State that may provide evidence relevant to customary international law.
Evidence of State Practice
Material demonstrating relevant State conduct for identifying or assessing a customary rule.
Evidence of Opinio Juris
Material indicating that States regard particular conduct as legally required, permitted or recognised.
Emerging Customary Rule
A proposed customary rule still developing through relevant State practice and legal acceptance.
Exceptional Practice
Conduct departing from an alleged customary rule whose legal significance must be assessed in context.
Legal Characterisation
The legal explanation a State gives for conduct, including whether it regards that conduct as required, permitted or unlawful.
Rule Identification
The process of determining the precise customary rule supported by the available evidence.
Rule Content and Scope
The specific obligations, permissions and factual circumstances established by an identified customary rule.
Statute of the International Court of Justice, Article 38
Article 38 provides the foundational framework for identifying the sources of international law applied by the Court and expressly refers to international custom as evidence of a general practice accepted as law. For this chapter, it establishes the starting point for understanding customary international law as a distinct source and links the concept directly to the two constituent elements developed throughout the chapter: general State practice and acceptance of that practice as law, or opinio juris.
Source: International Court of Justice — Statute of the International Court of Justice, Article 38(1).
Open Primary Source →North Sea Continental Shelf (1969)
This foundational judgment explains why customary international law cannot be established merely by pointing to repeated conduct or to a treaty provision. The Court examined the generality and consistency of State practice and whether States acted from a sense of legal obligation. It is therefore central to the F5 distinction between State practice and opinio juris and to the question whether a treaty rule has separately acquired customary status.
Source: International Court of Justice — Judgment, North Sea Continental Shelf, 20 February 1969.
Open Primary Source →Asylum (Colombia/Peru) (1950)
This judgment is an important authority on regional or local customary international law. The Court examined whether the alleged diplomatic asylum rule was supported by sufficiently established practice and whether that practice had the required legal character. It demonstrates that an asserted regional custom must be established through evidence relating to the States for whom the particular rule is claimed to exist.
Source: International Court of Justice — Judgment, Asylum (Colombia/Peru), 20 November 1950.
Open Primary Source →Right of Passage over Indian Territory (1960)
This case demonstrates how a local customary rule may arise from a particular historical relationship between States. The Court examined the conduct of Portugal and India and determined the precise scope of the customary right supported by the evidence. It is particularly useful for understanding that customary law may be particular rather than universal and that the content of a local rule must not be extended beyond the practice and legal acceptance actually established.
Source: International Court of Justice — Judgment, Right of Passage over Indian Territory, 12 April 1960.
Open Primary Source →Military and Paramilitary Activities in and against Nicaragua (1986)
This judgment is a major authority on the relationship between treaty obligations and customary international law. The Court examined customary rules concerning the use of force and non-intervention independently of parallel treaty obligations. It demonstrates that a rule may exist in both treaty and customary form, but that customary status must still be established through the relevant evidence of State practice and opinio juris.
Source: International Court of Justice — Merits Judgment, 27 June 1986.
Open Primary Source →Continental Shelf (Libyan Arab Jamahiriya/Malta) (1985)
This judgment is relevant to the identification and application of customary maritime rules and to the relationship between treaty provisions and customary international law. It illustrates the need to determine the precise customary rule applicable to the dispute rather than assuming that a treaty formulation or general maritime principle automatically supplies the complete customary rule.
Source: International Court of Justice — Judgment, Continental Shelf (Libyan Arab Jamahiriya/Malta), 3 June 1985.
Open Primary Source →Legality of the Threat or Use of Nuclear Weapons (1996)
This advisory opinion illustrates the difficulty of determining whether a developing area of international practice has matured into an established customary rule. The Court examined treaty provisions, State practice and legal positions concerning nuclear weapons. It is particularly relevant to distinguishing an emerging norm from an established rule and to the F5 requirement that contrary evidence must be considered rather than ignored.
Source: International Court of Justice — Advisory Opinion, 8 July 1996.
Open Primary Source →Barcelona Traction, Light and Power Company (1970)
This judgment is relevant to the identification of the legal character and scope of international obligations. The Court distinguished obligations owed to the international community as a whole from those arising in particular inter-State relationships. For F5, it provides supporting judicial material for understanding how international courts identify and describe rules of general international law, while also showing why the precise legal proposition must be identified before claiming that it is customary.
Source: International Court of Justice — Judgment, Barcelona Traction, Light and Power Company, 5 February 1970.
Open Primary Source →Jurisdictional Immunities of the State (2012)
This judgment provides a detailed example of evidence-based customary-law analysis. The Court considered national legislation, domestic judicial decisions and the legal positions of States when determining the customary status of State immunity. It is especially relevant to F5 because it shows why contrary practice must be examined and why a court must distinguish between conduct that rejects a rule, conduct treated as an exception, and conduct alleged to constitute a breach of an existing rule.
Source: International Court of Justice — Judgment, 3 February 2012.
Open Primary Source →Whaling in the Antarctic (2014)
This judgment is useful for demonstrating how an international court examines the legal and factual record before applying an international legal rule to State conduct. For F5, it reinforces the broader evidential method: a court must identify the applicable legal rule, examine the relevant State conduct and legal positions, and then determine whether the facts support the legal conclusion being asserted.
Source: International Court of Justice — Judgment, 31 March 2014.
Open Primary Source →Questions relating to the Obligation to Prosecute or Extradite (2012)
This judgment is relevant to analysing the relationship between treaty obligations and questions of customary international law. It demonstrates why a court must identify the actual legal basis of an obligation and avoid assuming that a treaty obligation and a customary rule necessarily have identical content or scope.
Source: International Court of Justice — Judgment, 20 July 2012.
Open Primary Source →Question of the Delimitation of the Continental Shelf between Nicaragua and Colombia beyond 200 nautical miles (2023)
This modern judgment illustrates the final stage of the F5 customary-law method: identifying the precise rule, determining its content and scope, and applying it to the facts. It demonstrates why a court must not extend an established customary rule beyond what the evidence and the rule's actual legal content support.
Source: International Court of Justice — Judgment, 13 July 2023.
Open Primary Source →Vienna Convention on the Law of Treaties (1969)
The Vienna Convention provides the principal codification framework for the law of treaties between States, including treaty formation, consent, reservations, interpretation, observance, application, amendment, invalidity, termination and suspension. For F5, it is particularly important when analysing how treaty obligations relate to customary international law and whether a treaty provision may also reflect or contribute to a customary rule.
Source: United Nations / United Nations Treaty Collection — Vienna Convention on the Law of Treaties, 1969.
Open Primary Source →ILC Draft Conclusions on Identification of Customary International Law (2018)
This is the principal methodological reference for the F5 chapter. The International Law Commission's sixteen draft conclusions address the two constituent elements, assessment of evidence, forms and assessment of State practice, generality, opinio juris, treaties, resolutions of international organisations and intergovernmental conferences, judicial decisions, teachings, the persistent-objector doctrine and particular customary international law. It therefore provides a systematic framework for the evidence-based method taught throughout this chapter.
Source: United Nations International Law Commission — Draft Conclusions on Identification of Customary International Law with Commentaries, 2018.
Open Primary Source →United Nations Charter
The United Nations Charter provides the institutional framework within which the International Court of Justice operates and contains provisions central to several areas in which customary international law is assessed, including the prohibition of the use of force and the international legal system governing relations between States. In F5, the Charter is relevant when considering how treaty provisions, State practice and opinio juris may interact in the development and identification of customary rules.
Source: United Nations — Charter of the United Nations.
Open Primary Source →Fisheries (United Kingdom v. Norway) (1951)
This judgment provides an important example of how long-standing State practice, geographical circumstances and the reactions of other States may be assessed together. It demonstrates that State practice is not simply a numerical count of acts. The legal significance of conduct depends upon its context and upon how other States respond to the claimed rule, making the case useful for F5's treatment of consistency, representativeness and contrary practice.
Source: International Court of Justice — Judgment, Fisheries (United Kingdom v. Norway), 18 December 1951.
Open Primary Source →North Sea Continental Shelf — Persistent Objection and State Practice
This authority is retained because the North Sea judgment is important to the broader assessment of State practice and legal acceptance during the development of customary international law. It should, however, be read alongside the modern ILC treatment of the persistent-objector doctrine, which specifically addresses the conditions under which a State that persistently objects to an emerging customary rule may avoid its application.
Source: International Court of Justice — North Sea Continental Shelf, Judgment, 20 February 1969.
Open Primary Source →North Sea Continental Shelf Cases
Court: International Court of Justice
Facts
Germany, Denmark and the Netherlands disputed the principles governing the delimitation of their continental shelf boundaries in the North Sea. Denmark and the Netherlands relied in part on the 1958 Continental Shelf Convention and its equidistance principle. Germany was not party to the Convention provision relied upon, so the dispute required the Court to consider whether the equidistance principle nevertheless bound Germany as customary international law. The case therefore required the Court to distinguish a treaty obligation from an independently existing customary rule.
Source / Instrument
North Sea Continental Shelf, Judgments, International Court of Justice, 20 February 1969; Convention on the Continental Shelf, 1958.
Legal Issue
Whether the equidistance principle had become binding on Germany as a rule of customary international law despite Germany not being party to the relevant treaty provision, and what evidence was required to establish the customary character of the rule.
Rule
A customary rule requires the necessary general practice of States together with acceptance of that practice as law. Repeated conduct alone is insufficient, and the existence of a treaty provision does not automatically establish an equivalent customary obligation for States outside the treaty relationship.
Judicial Reasoning
The Court examined whether the relevant State practice was sufficiently extensive and representative and whether States followed the practice because they considered themselves legally bound to do so. It rejected the proposition that a treaty rule automatically becomes customary merely because States have accepted it in a treaty. The Court also stressed that the evidence must demonstrate the legal character of the practice rather than simply showing that States have behaved in a similar manner. The analysis therefore illustrates the separation between observable conduct and opinio juris.
Application
The Court examined the practice relied upon in support of the equidistance principle and considered whether States had treated that method as legally obligatory independently of the Convention. The evidence did not demonstrate the necessary combination of general practice and acceptance as law. The Court consequently did not treat the treaty provision as automatically binding Germany through customary law.
Result
The Court concluded that the equidistance principle had not been shown on the evidence before it to have become binding upon Germany as a rule of customary international law.
Why it matters
This is one of the principal authorities for the F5 method of identifying custom. It demonstrates why a court must separately examine State practice and opinio juris, why treaty participation cannot simply be equated with customary obligation, and why the quality, consistency and legal character of evidence matter. It also provides a foundation for analysing whether an alleged customary rule existed at the relevant time.
Limits
The judgment concerned continental shelf delimitation and the evidence available in 1969. It should not be treated as establishing a universal numerical threshold for State practice or as requiring identical evidence in every area of international law.
Military and Paramilitary Activities in and against Nicaragua
Court: International Court of Justice
Facts
Nicaragua brought proceedings against the United States concerning military and paramilitary activities. The dispute involved the use of force, intervention and related principles of international law. The United States also relied upon arguments concerning treaty obligations and jurisdiction, requiring the Court to consider the relationship between treaty law and customary international law.
Source / Instrument
Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America), Merits, Judgment, International Court of Justice, 27 June 1986.
Legal Issue
Whether rules concerning the prohibition of the use of force and non-intervention existed as customary international law independently of treaty obligations and how the Court should identify those rules.
Rule
A rule may exist simultaneously as a treaty obligation and as a rule of customary international law. The customary rule must nevertheless be identified from the evidence of State practice and opinio juris rather than assumed solely from the existence of the treaty.
Judicial Reasoning
The Court examined State practice and official statements concerning the use of force and intervention. It considered whether States regarded the relevant principles as legally binding and distinguished customary obligations from obligations arising directly from treaty law. The Court also recognised that conduct inconsistent with a rule does not automatically demonstrate rejection of the rule; the legal explanation given by the State for its conduct can be important. This illustrates the F5 requirement to examine both positive and negative evidence.
Application
The Court assessed the conduct and legal positions of States concerning force and intervention and considered whether the evidence demonstrated the customary character of the relevant rules. It did not treat the parallel treaty framework as preventing customary rules from operating independently.
Result
The Court recognised relevant customary rules concerning the use of force and non-intervention and applied them independently of the treaty framework.
Why it matters
This case is central to understanding the relationship between treaties and custom. It demonstrates that a treaty provision may coexist with an independent customary rule and that the customary rule must still be established through the appropriate evidence. It also illustrates how State conduct, statements and legal explanations can be combined when determining opinio juris.
Limits
The findings concerned the particular customary rules and evidence considered in the dispute. The case does not mean that every treaty provision automatically creates or confirms an identical customary rule.
Asylum (Colombia/Peru)
Court: International Court of Justice
Facts
Colombia granted diplomatic asylum in its embassy in Lima to a Peruvian political figure. Colombia argued that a regional or local customary rule governed diplomatic asylum and supported its position by reference to alleged practice among Latin American States. Peru disputed the existence and legal effect of the claimed rule.
Source / Instrument
Asylum (Colombia/Peru), Judgment, International Court of Justice, 20 November 1950.
Legal Issue
Whether a sufficiently established regional or local customary rule concerning diplomatic asylum existed between the States concerned.
Rule
A particular or regional customary rule requires evidence demonstrating the relevant practice among the States concerned together with acceptance of that practice as law. General assertions about regional practice cannot substitute for evidence establishing the particular rule claimed.
Judicial Reasoning
The Court examined the historical material relied upon by Colombia and considered whether the alleged practice was sufficiently consistent and whether it had been accepted as legally obligatory. The Court did not treat isolated examples, diplomatic arrangements or general statements about regional tradition as automatically establishing a customary rule. The case demonstrates that the more specific the alleged rule, the more carefully the evidence must be matched to the proposition being asserted.
Application
The Court compared the alleged regional rule with the actual practice and legal positions of the States concerned. It found that the evidence did not demonstrate a sufficiently established customary entitlement of the scope claimed by Colombia.
Result
The Court found that the claimed regional customary rule concerning diplomatic asylum had not been established on the evidence presented.
Why it matters
This case provides a practical example of regional and local custom. It teaches that a court must identify exactly which States are said to be bound, determine whether their practice is sufficiently consistent, and establish that the practice is accepted as law. It also illustrates why a court should not transform a collection of historical examples into a legal rule without examining their legal character.
Limits
The case concerned diplomatic asylum and the particular regional evidence presented to the Court. Its reasoning should not be treated as requiring identical evidence for every form of regional or local customary law.
Right of Passage over Indian Territory
Court: International Court of Justice
Facts
Portugal claimed a right of passage between its territory and Portuguese enclaves in India. India disputed the existence and scope of the claimed right. The dispute required the Court to examine the historical conduct of the two States and whether that conduct had generated a local customary rule.
Source / Instrument
Right of Passage over Indian Territory, Judgment, International Court of Justice, 12 April 1960.
Legal Issue
Whether a local customary rule concerning passage had developed between Portugal and India through their conduct and the legal significance attributed to that conduct.
Rule
Local custom may arise where a sufficiently established practice between the States concerned is accepted as legally binding. The content of such a rule must be determined from the evidence and cannot be extended beyond what the established practice supports.
Judicial Reasoning
The Court examined the historical relationship between the parties and distinguished different forms and purposes of passage. It considered whether the conduct of the parties demonstrated an accepted legal practice rather than merely a pattern of tolerated behaviour. The analysis shows that customary rules must be identified with precision and that evidence may establish a limited rule without establishing every broader proposition claimed by a party.
Application
The Court assessed the actual historical practice and its circumstances and distinguished the categories of passage for which the evidence provided support. It did not simply assume that every form of passage had acquired the same customary status.
Result
The Court recognised a limited local customary right of passage in specified circumstances.
Why it matters
This case is important because it demonstrates that customary international law can operate at a local level and that the court must determine the precise content and scope of the rule actually established. It is particularly useful when applying F5's evidence-based method to a dispute where the alleged rule is narrower than a general principle.
Limits
The finding concerned the particular local relationship between Portugal and India and the specific forms of passage supported by the evidence.
Legality of the Threat or Use of Nuclear Weapons
Court: International Court of Justice
Facts
The General Assembly requested an advisory opinion concerning the legality under international law of the threat or use of nuclear weapons. The Court was required to consider treaty obligations, customary rules, State practice and the legal positions expressed by States concerning nuclear weapons.
Source / Instrument
Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, International Court of Justice, 8 July 1996.
Legal Issue
What did the available treaty and customary evidence establish concerning the legality of the threat or use of nuclear weapons and whether a general customary prohibition had developed?
Rule
The existence and content of a customary rule must be established from the relevant evidence, including State practice and opinio juris. Evidence of an emerging norm or policy objective must not automatically be treated as proof that a complete customary prohibition already exists.
Judicial Reasoning
The Court examined the conduct and legal positions of States, together with relevant treaty provisions and institutional material. It considered both evidence supporting restrictions and evidence showing continued reliance upon nuclear weapons by some States. The reasoning demonstrates why a court must assess supporting and contrary evidence and distinguish an emerging legal development from a rule whose customary status is already established.
Application
The Court assessed the totality of the evidence rather than relying upon isolated statements or conduct. It considered whether the evidence demonstrated the necessary legal acceptance of a comprehensive customary prohibition.
Result
The Court did not find that the evidence established a comprehensive and universally applicable customary prohibition on the threat or use of nuclear weapons in the circumstances addressed by the opinion.
Why it matters
This case demonstrates how a court should deal with a contested and developing area of international law. It is particularly valuable for F5 because it shows that widespread concern, policy statements and developing practice must still be distinguished from an established customary rule.
Limits
The opinion addressed the specific questions submitted by the General Assembly and the evidence available to the Court at that time. Later developments must be assessed separately.
Jurisdictional Immunities of the State
Court: International Court of Justice
Facts
Germany brought proceedings against Italy concerning Italian judicial measures involving Germany and claims arising from conduct during the Second World War. The dispute required the Court to consider whether rules concerning State immunity formed part of customary international law and how national judicial practice should be evaluated.
Source / Instrument
Jurisdictional Immunities of the State (Germany v. Italy: Greece intervening), Judgment, International Court of Justice, 3 February 2012.
Legal Issue
Whether the relevant rules of State immunity formed part of customary international law and how national legislation, judicial decisions and contrary practice affected that assessment.
Rule
Customary-law identification requires assessment of State practice together with opinio juris. National legislation and judicial decisions may provide important evidence, but their significance must be assessed alongside the wider international record. Contrary practice must also be considered rather than ignored.
Judicial Reasoning
The Court examined national legislation, domestic judicial decisions and the legal positions of States. It considered whether departures from the asserted rule represented rejection of the rule or were treated by States as exceptions or unlawful conduct. The analysis demonstrates why a court should examine the legal explanation surrounding conduct rather than simply count instances of compliance and non-compliance.
Application
The Court compared the relevant national and international material and assessed the legal character of the practice. It concluded that departures did not by themselves displace the customary rule where the broader evidence demonstrated acceptance of State immunity.
Result
The Court concluded that the relevant rule of State immunity formed part of customary international law in the circumstances before it.
Why it matters
This case is especially useful for the evidential methodology taught in F5. It shows how national legislation, national judicial decisions, State positions and contrary practice can be brought together in a customary-law analysis. It also demonstrates the difference between breach, exception and rejection of a customary rule.
Limits
The case concerned State immunity and the particular exceptions and circumstances raised by the parties. Its treatment of national material must be understood in that specific context.
Fisheries (United Kingdom v. Norway)
Court: International Court of Justice
Facts
The United Kingdom challenged Norway's method of drawing straight baselines along its deeply indented coastline for measuring the breadth of its territorial sea. The dispute required consideration of Norway's long-standing practice, the geographical circumstances and the reactions of other States.
Source / Instrument
Fisheries (United Kingdom v. Norway), Judgment, International Court of Justice, 18 December 1951.
Legal Issue
Whether Norway's method was consistent with international law and what significance should be given to its long-standing practice and the responses of other States.
Rule
The legal significance of State conduct must be assessed in its factual and geographical context. The reactions of other States may be relevant to determining whether conduct has been accepted, contested or acquiesced in as a matter of international law.
Judicial Reasoning
The Court considered Norway's historical practice, the geographical character of the coastline and the international response to the method used. It did not analyse the conduct in isolation from its circumstances. The case demonstrates why a court must examine the surrounding context and not assume that a practice can be understood merely by observing the physical conduct itself.
Application
The Court assessed Norway's particular practice together with the geographical circumstances and the conduct of other States. The evidence supported the legality of Norway's method in the circumstances considered.
Result
The Court upheld Norway's method of drawing the baselines in the circumstances examined.
Why it matters
The case is useful for F5 because it illustrates the importance of context, sustained practice and the reactions of other States. It helps students understand that State practice is not simply a numerical count of governmental acts but must be evaluated for its legal significance and surrounding circumstances.
Limits
The judgment concerned Norway's particular coastline and the evidence available concerning that geographical and historical situation.
Question of the Delimitation of the Continental Shelf between Nicaragua and Colombia beyond 200 Nautical Miles
Court: International Court of Justice
Facts
Nicaragua and Colombia disputed maritime entitlements beyond 200 nautical miles. The dispute required the Court to determine the applicable international legal framework and to consider the scope of continental shelf rights under customary international law.
Source / Instrument
Question of the Delimitation of the Continental Shelf between Nicaragua and Colombia beyond 200 nautical miles, Judgment, International Court of Justice, 13 July 2023.
Legal Issue
What did customary international law establish concerning continental shelf entitlements beyond 200 nautical miles and how should the relevant customary rules be applied to the maritime circumstances?
Rule
A court must identify the precise customary rule supported by the evidence and then determine its content and scope before applying it to the facts. An established general principle does not automatically establish every more specific proposition asserted by a party.
Judicial Reasoning
The Court considered the applicable customary rules concerning continental shelf entitlements and examined the legal framework governing the competing claims. Its reasoning illustrates the importance of defining the precise proposition said to be customary before asking whether that proposition applies to the facts. The case therefore demonstrates the final stages of the F5 methodology: identify the rule, establish its content, determine its scope and apply it without extending the rule beyond its legal basis.
Application
The Court applied the identified customary rules to the maritime claims and distinguished the established legal entitlement from the broader entitlement asserted by Nicaragua.
Result
The Court rejected Nicaragua's claimed entitlement beyond 200 nautical miles in the circumstances presented.
Why it matters
This is a modern example of customary-law reasoning in practice. It demonstrates that a court must do more than establish that a customary principle exists. It must determine exactly what the rule requires, whether it covers the facts before the Court and whether the requested legal consequence is actually supported by the rule.
Limits
The judgment concerned the particular maritime claims and legal questions submitted by Nicaragua and Colombia and should not be generalised beyond the rules and circumstances addressed.
Customary International Law: Establishing, Evaluating and Applying a Customary Rule
Instructions
Read the scenario and each task carefully. Formulate the precise legal proposition before assessing evidence. Distinguish State practice from opinio juris and legal obligation from policy, habit or convenience. Assess supporting and contrary evidence fairly. Treat treaties, resolutions, judicial decisions and scholarship according to their proper evidential or subsidiary role. Write your own legal reasoning, submit it, compare it with the Model Answer and Marking Guide, self-assess against the criteria, enter the score and record it. Clear Answer clears only your typed response.
Overall Scenario
The Republic of Arctavia and the State of Bellmare are coastal States bordering the Meridian Sea. For more than twenty years, Arctavia has required foreign research vessels entering a defined environmental protection zone to give advance notification and specified scientific information. Arctavia claims that this requirement is customary international law. Bellmare is not party to the 2010 Meridian Environmental Convention, whose Article 14 contains similar wording. Bellmare argues that Arctavia is relying on domestic policy and treaty obligations that cannot automatically bind a non-party. The evidence is mixed. Twelve States have legislation containing notification requirements of varying scope. Six are parties to the 2010 Convention, and three expressly state that their legislation implements that treaty. Four non-party States have also applied notification requirements, and two expressly describe the requirement as customary international law. Several States have issued diplomatic statements describing advance notification as legally required for activities presenting serious environmental risks. Other States describe notification as policy, cooperation or administrative convenience. In three incidents, States protested Arctavia. One described the measures as politically excessive. One alleged violation of customary international law. One stated that no general customary rule could be established. The International Maritime Organisation adopted a resolution supporting advance notification. Several States explained that they supported it as policy rather than as existing customary law. One national court called the requirement customary; another rejected that conclusion. Academic commentators are divided. The dispute concerns whether the alleged rule existed by 1 January 2020 and, if so, whether it requires the precise notification and information-disclosure obligation asserted by Arctavia.
How to Complete the Assessment
Read each task and legal question carefully. Write your own legal reasoning in the answer box, then select Submit Answer. Your submitted answer will then be followed by the Correct Answer so that you can compare your reasoning with the model legal analysis.
Task 1
Formulate the Alleged Customary Rule
Task Scenario
The Court must identify exactly what Arctavia claims international law requires before testing the evidence.
Legal Question
Formulate the precise customary rule alleged by Arctavia. Separate the general notification obligation from the proposed information-disclosure requirement, scope and relevant date.
Model Answer & Marking Guide
Model Answer: The alleged rule concerns advance notification for defined environmentally sensitive research activity, together with any separate information-disclosure obligation, applicable to the relevant States and circumstances by 1 January 2020.
Legal Analysis: A customary-law inquiry must begin with a precise legal proposition. Evidence supporting a broad principle does not automatically establish every detailed obligation, threshold, exception or procedure.
Guidance: State the narrowest formulation that allows each alleged element to be tested separately.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- Identifies custom as the alleged legal source — 1 mark
- Formulates a precise obligation — 2 marks
- Separates notification from information disclosure — 1 mark
- Identifies scope and circumstances — 2 marks
- Recognises the relevant date — 1 mark
Task 2
State Practice
Task Scenario
The record contains legislation, actual enforcement and governmental measures from numerous States.
Legal Question
Identify the principal State-practice evidence and explain how the Court should assess its relevance and independence.
Model Answer & Marking Guide
Model Answer: The legislation and enforcement measures are relevant State practice. Their weight varies according to purpose and legal context. Treaty implementation may show treaty compliance rather than independent custom.
Legal Analysis: State practice includes attributable official conduct and acts such as legislation, administrative measures, enforcement, diplomatic conduct and other authoritative governmental acts. The Court must ask why each act occurred and what precise proposition it supports.
Guidance: Identify the source, authority, purpose and context of each important item. Avoid treating treaty implementation as automatically independent customary practice.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- Identifies relevant forms of State practice — 2 marks
- Addresses attribution and official character — 1 mark
- Assesses purpose and context — 2 marks
- Recognises the importance of independent practice — 2 marks
Task 3
Generality, Consistency and Representativeness
Task Scenario
The twelve legislative examples contain different versions of notification, while practice is spread unevenly among States.
Legal Question
Assess whether the practice is sufficiently general, consistent and representative for the precise rule alleged.
Model Answer & Marking Guide
Model Answer: The record may support a broad pattern but variations in scope weaken the claim to the precise rule unless the legal positions and practice converge on those details.
Legal Analysis: Customary law is not established by mechanical numerical counting. The Court assesses breadth, geographical distribution, diversity, consistency and the relevance of States particularly connected with the subject.
Guidance: Distinguish evidence supporting a general notification principle from evidence establishing every detailed requirement.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- Explains generality — 2 marks
- Assesses consistency — 2 marks
- Assesses representativeness — 2 marks
- Distinguishes general support from detailed proof — 1 mark
Task 4
Opinio Juris
Task Scenario
Some States expressly describe notification as required by customary international law, while others act for policy or convenience.
Legal Question
Determine what evidence supports opinio juris and distinguish legal acceptance from political preference, habit or convenience.
Model Answer & Marking Guide
Model Answer: The clearest evidence comes from States expressly identifying the requirement as customary law and official statements expressly invoking international legal obligation. Policy or convenience alone is insufficient.
Legal Analysis: Opinio juris concerns the legal character States attribute to conduct. Similar conduct may arise from convenience, reciprocity or policy and therefore does not automatically demonstrate legal acceptance.
Guidance: Look for specific legal explanations connected to the precise rule. Multiple motives can coexist, but legal acceptance must still be evidenced.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- Defines opinio juris accurately — 2 marks
- Identifies direct legal acceptance — 2 marks
- Distinguishes policy and convenience — 2 marks
- Recognises that motives may coexist — 1 mark
Task 5
Diplomatic Protests and Official Statements
Task Scenario
The three protests use different language and therefore do not have identical legal significance.
Legal Question
Assess the three protests and explain what they demonstrate about the legal positions of the States concerned.
Model Answer & Marking Guide
Model Answer: The political complaint is weaker evidence of opinio juris. The protest alleging customary illegality supports legal acceptance. The express rejection of custom is significant contrary evidence.
Legal Analysis: A protest expressly alleging breach of international law can provide strong evidence of opinio juris. A political complaint may show opposition without asserting a legal rule. An express rejection of the alleged rule is contrary evidence.
Guidance: Classify each protest according to the legal proposition actually expressed rather than treating all protests as equivalent.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- Distinguishes political and legal protest — 2 marks
- Identifies evidence of opinio juris — 2 marks
- Identifies contrary legal rejection — 1 mark
- Explains contextual assessment — 1 mark
Task 6
Contrary Practice, Breach and Rejection
Task Scenario
States have acted inconsistently with Arctavia's proposed rule, but their legal explanations differ.
Legal Question
Explain how the Court should determine whether contrary conduct represents breach, an exception, disagreement about scope, or rejection of the rule.
Model Answer & Marking Guide
Model Answer: The Court must distinguish conduct accompanied by acceptance of the rule from conduct accompanied by express rejection. A claimed exception or scope dispute is different from denial of the rule itself.
Legal Analysis: Contrary conduct does not automatically disprove custom. A State may breach a rule while accepting it, claim an exception, dispute application, or expressly reject the rule. The explanation accompanying conduct is critical.
Guidance: Ask what each State says the conduct means and how other States respond.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- Distinguishes breach from rejection — 2 marks
- Addresses exceptions and scope — 2 marks
- Uses State explanations — 1 mark
- Addresses responses of other States — 2 marks
Task 7
National Legislation and National Judgments
Task Scenario
Domestic legislation and two national courts point in different directions.
Legal Question
Evaluate the evidential significance of national legislation and national judicial decisions.
Model Answer & Marking Guide
Model Answer: The domestic material is relevant evidence. Treaty-implementing legislation has reduced value for independent custom. The conflicting judgments demonstrate competing State legal positions and must be assessed with the wider record.
Legal Analysis: National legislation is State practice and may reveal legal understanding when its purpose or explanation connects it to international law. National judgments can reveal a State institutional legal position but do not themselves create universal custom.
Guidance: Consider whether legislation implements a treaty, domestic policy or an asserted international legal obligation. Assess the reasoning and authority of each judgment.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- Treats legislation as potential State practice — 2 marks
- Assesses treaty implementation — 2 marks
- Assesses national judicial decisions — 2 marks
- Rejects automatic universal effect — 1 mark
Task 8
Treaties and Customary Law
Task Scenario
The 2010 Convention contains wording similar to Arctavia's proposed rule, but Bellmare is not a party.
Legal Question
Explain how the Court should determine whether Article 14 reflects existing custom, contributes to crystallisation, or remains a treaty-only obligation.
Model Answer & Marking Guide
Model Answer: Article 14 is relevant evidence but does not automatically bind Bellmare. The Court must determine independently whether the same rule exists as customary law.
Legal Analysis: A treaty may codify an existing customary rule, contribute to crystallisation or development, or create obligations only between treaty parties. Treaty participation and compliance do not automatically establish independent custom.
Guidance: Look for independent practice and legal acceptance, especially from non-parties and from States expressly describing the provision as existing law.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- Distinguishes treaty and customary obligations — 2 marks
- Explains codification — 1 mark
- Explains crystallisation or development — 2 marks
- Identifies independent evidence — 2 marks
Task 9
Specially Affected States
Task Scenario
The dispute concerns a specialised maritime and environmental activity in which some States have unusually extensive direct involvement.
Legal Question
Explain the evidential relevance of specially affected States.
Model Answer & Marking Guide
Model Answer: Particularly affected States may carry particular evidential weight, but their practice must be considered with the wider body of relevant State practice and opinio juris.
Legal Analysis: States particularly involved in the subject matter may have particular evidential importance because they have greater opportunities to develop relevant practice and legal positions. This does not give them an automatic veto or exclusive control over custom.
Guidance: Explain why direct involvement matters while keeping the assessment within the broader international record.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- Identifies special evidential relevance — 2 marks
- Explains direct involvement — 2 marks
- Rejects exclusive control or automatic veto — 1 mark
- Places evidence in wider record — 1 mark
Task 10
Persistent Objection and Time
Task Scenario
The parties dispute whether the customary rule existed by 1 January 2020 and rely on objections made at different times.
Legal Question
Explain how the Court should assess persistent objection and the temporal question.
Model Answer & Marking Guide
Model Answer: The Court must determine whether Bellmare clearly and consistently objected while the rule was developing and whether the customary requirements were satisfied by 1 January 2020.
Legal Analysis: Persistent objection requires clear and consistent objection during formation. The Court must reconstruct the evidence chronologically and cannot automatically project a later rule backwards.
Guidance: Identify when the objection began, what it rejected, whether it continued, and whether formation had already occurred.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- States persistent-objection requirements — 2 marks
- Addresses timing and continuity — 2 marks
- Applies the relevant date — 1 mark
- Separates later development from earlier law — 1 mark
Task 11
International Organisations and Resolutions
Task Scenario
The Maritime Organisation resolution was adopted by a large majority, but some States expressly supported it only as policy.
Legal Question
Assess the evidential significance of the resolution and the explanations of vote.
Model Answer & Marking Guide
Model Answer: The resolution is relevant evidence, but its customary significance depends on State positions, explanations of vote, adoption circumstances and subsequent practice.
Legal Analysis: Institutional resolutions can record State positions and contribute evidence, but the resolution itself is not automatically identical to universal State practice or opinio juris. Voting context and explanations matter.
Guidance: Examine whether States supported the proposition as existing law, future development or policy.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- Characterises the resolution correctly — 2 marks
- Assesses State positions and explanations — 2 marks
- Distinguishes policy from legal acceptance — 1 mark
- Considers subsequent practice — 1 mark
Task 12
Judicial Decisions and Scholarly Writings
Task Scenario
The parties rely on ICJ decisions and divided academic commentary.
Legal Question
Explain the role and limits of international judicial decisions and scholarly writings in identifying custom.
Model Answer & Marking Guide
Model Answer: ICJ decisions are important subsidiary judicial reasoning, while scholarship is a subsidiary means of identifying and organising evidence. Neither independently creates the customary rule.
Legal Analysis: Judicial decisions can organise and assess State practice and opinio juris and provide persuasive reasoning. Scholarship can locate and analyse evidence. Neither replaces the underlying customary elements.
Guidance: Return from judicial or academic authority to the evidence on which the legal proposition depends.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- Explains the role of judicial decisions — 2 marks
- Explains the subsidiary role of scholarship — 2 marks
- Returns to underlying State evidence — 1 mark
- Distinguishes authority from evidence — 1 mark
Task 13
Emerging, Regional or Insufficient Custom
Task Scenario
The evidence is stronger for a broad notification principle than for the exact information-disclosure requirement.
Legal Question
Determine whether the evidence establishes a general customary rule, an emerging practice, a regional or local rule, or no rule at all.
Model Answer & Marking Guide
Model Answer: The Court may recognise a narrower established customary core, an emerging practice, a regional or local rule, or insufficient evidence. It should not extend a customary finding beyond the evidence.
Legal Analysis: The Court is not restricted to accepting or rejecting the entire formulation. A narrower customary core may be established while additional details remain unsupported. The Court must also distinguish general from regional or local custom and established law from developing practice.
Guidance: State the narrowest conclusion justified by the evidence.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- Distinguishes established and emerging custom — 2 marks
- Distinguishes general and regional or local custom — 1 mark
- Recognises a narrower possible rule — 2 marks
- States the evidence-based approach — 2 marks
Task 14
Apply the Rule to the Dispute
Task Scenario
Assume the Court finds that a customary obligation of advance notification for defined environmentally sensitive research activity existed by 1 January 2020.
Legal Question
Apply that rule to Bellmare's conduct and determine whether breach is established. Keep jurisdiction, responsibility and remedy analytically separate.
Model Answer & Marking Guide
Model Answer: If Bellmare's conduct falls within the established rule and the factual requirements are proved, failure to provide required notification may constitute breach. Jurisdiction, responsibility and remedy require separate analysis.
Legal Analysis: After establishing a rule, the Court must determine its scope and application to proven facts. Existence of a customary rule does not itself establish jurisdiction, attribution, responsibility or remedy.
Guidance: Identify the conduct, determine whether it falls within the rule, then analyse breach separately from other legal consequences.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- Identifies the established rule and scope — 2 marks
- Applies the rule to the facts — 2 marks
- Considers qualifications or exceptions — 1 mark
- Separates breach from jurisdiction and remedy — 2 marks
Task 15
Final Judicial Finding
Task Scenario
The Court must now deliver its final finding on the existence, content, scope and application of the alleged customary rule.
Legal Question
Write a reasoned judicial conclusion addressing State practice, opinio juris, generality, consistency, representativeness, contrary evidence, treaty context, time and the precise scope of any rule established.
Model Answer & Marking Guide
Model Answer: The Court should recognise only the customary rule established by the evidence. If the evidence establishes notification but not the claimed information-disclosure requirement, the judgment should state that narrower conclusion and explain why. If the evidence is insufficient, the Court should say so rather than manufacture certainty.
Legal Analysis: A proper finding makes the evidential chain visible. The Court must explain what evidence establishes practice, what demonstrates legal acceptance, how contrary material is treated, why treaty and institutional material has its particular weight, and why the final rule is no broader than the evidence supports.
Guidance: Use the sequence: precise rule, relevant States and period, practice, generality and consistency, opinio juris, contrary evidence and objections, treaties and supporting authorities, final scope, application.
Self-Assessment: Compare your answer with the model answer and criteria below, then award yourself the marks you believe you have earned.
- States a clear final conclusion — 1 mark
- Synthesises practice and opinio juris — 2 marks
- Addresses contrary evidence and treaty context — 1 mark
- Addresses time and precise scope — 1 mark
- Provides reasoned judicial justification — 2 marks
Self-Assessment Score: 0 / 100 (0%)
Assessment Criteria
- Precise formulation of the alleged customary rule.
- Accurate distinction between State practice and opinio juris.
- Evidence-based assessment of generality, consistency and representativeness.
- Proper treatment of contrary practice, objections and persistent objection.
- Correct treatment of specially affected States.
- Correct distinction between treaty obligations and independent customary obligations.
- Proper use of international organisation material, judicial decisions and scholarly writings.
- Attention to temporal and geographical scope.
- Ability to distinguish established custom from emerging or insufficient practice.
- Ability to apply a customary rule while separating existence, breach, jurisdiction, responsibility and remedy.
- Clear and reasoned judicial conclusion.
International Court of Justice — Judgments and Advisory Opinions
Official ICJ materials containing decisions relevant to customary international law.
Open Resource →International Court of Justice — Statute
Official text of the ICJ Statute, including Article 38.
Open Resource →United Nations Treaty Collection
Official United Nations treaty materials useful for distinguishing treaty practice from customary law.
Open Resource →United Nations International Law Commission
Official materials concerning the identification and development of international law.
Open Resource →United Nations Audiovisual Library of International Law
Official lectures and materials on international legal subjects.
Open Resource →United Nations Charter
Official United Nations framework relevant to international legal relations.
Open Resource →Vienna Convention on the Law of Treaties
Official treaty-law framework relevant to distinguishing treaty and customary obligations.
Open Resource →International Law Commission — Conclusions on Identification of Customary International Law
Official UN International Law Commission material directly concerning identification of customary international law.
Open Resource →ICJ — North Sea Continental Shelf Cases
Official ICJ case material concerning State practice and opinio juris.
Open Resource →ICJ — Military and Paramilitary Activities in and against Nicaragua
Official ICJ case material concerning customary international law.
Open Resource →ICJ — Asylum Case
Official ICJ case material concerning local custom and evidential requirements.
Open Resource →ICJ — Right of Passage over Indian Territory
Official ICJ case material concerning local customary international law.
Open Resource →ICJ — Nuclear Weapons Advisory Opinion
Official ICJ material concerning customary rules and State practice.
Open Resource →ICJ — Jurisdictional Immunities
Official ICJ material concerning State practice and customary international law.
Open Resource →United Nations Legal Affairs
Official UN legal materials and international-law resources.
Open Resource →UN Digital Library
Official UN research materials containing State statements and international legal documents.
Open Resource →United Nations Documentation Research Guide
Official guidance for locating UN documents and State positions.
Open Resource →International Law Reports
Legal reporting resource useful for comparative study of international judicial decisions.
Open Resource →Oxford Public International Law
Academic reference material useful for researching international legal doctrine.
Open Resource →Max Planck Encyclopedia of Public International Law
Academic reference material for further study of customary international law.
Open Resource →ILC Draft Conclusions on Identification of Customary International Law with Commentaries
Detailed official methodology addressing general practice, opinio juris, forms of practice, treaties, resolutions, judicial decisions, persistent objection and particular customary international law.
Open Resource →UN General Assembly Resolution 73/203 — Identification of Customary International Law
General Assembly resolution taking note of the ILC conclusions on identification of customary international law and bringing them to the attention of States and those required to identify customary rules.
Open Resource →CHAPTER EDUCATIONAL VIDEO
Watch and Learn
These educational videos are provided through authorised publisher/platform embedding mechanisms. The Court does not download or re-host the video files.
A Guide to the Most Elusive Source of International Law
University of Oxford discussion of the identification and content of customary international law, including methodology, evidence, State practice and opinio juris.
Teaching topic:
Use and source information: