Lessons on Article 38(1) of the ICJ, treaties, customary international law, general principles of international law and soft law.

Nicaragua vs United States: An Analysis of Jurisprudence on Customary International Law

© Ruwanthika Gunaratne and Public International Law at, 2008 – present. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Ruwanthika Gunaratne and Public International Law with appropriate and specific direction to the original content.

Case: Case Concerning the Military and Paramilitary Activities In and Against Nicaragua (Nicaragua vs United States) 

Year of Decision: 1986. Court: ICJ.

NB: This blog post will discuss only the court’s deliberations on customary international law. For a brief summary of the facts and matters on the definition of an armed attack, use of force, self-defense and intervention, the reader is referred to a previous post. 

Overview: The case involved military and paramilitary activities conducted by, or with the assistance of, the United States against Nicaragua from 1981 to 1984. Due to a multilateral treaty reservation of the United States (hereinafter called the Vandenberg reservation), the Court was compelled to base its findings only on customary and general principles of international law. As a result, the Nicaragua case developed significant jurisprudence on clarifying customary international law on the use of force and non-intervention, elements necessary to form customary international law and the relationship between the latter and treaty law. Controversial aspects of the decision included the court’s methodology used to determine that the principle of non-intervention had attained customary law status, the court’s reliance on UN resolutions as a source of opinio juris and the court’s reliance on multilateral treaties to determine customary international law in face of the Vandenberg reservation. 

Recommendation: The Nicaragua case contains in-depth discussions on the relationship between treaty and customary international law. Students may wish to read this post on the relationship before reading the synopsis of the case.


In the Nicaragua case, the ICJ discussed:

  • The competence of the ICJ to give its determination based on customary international law in the face of the Vandenberg reservation of the United States.
  • The relationship between treaty law and customary international law.
  • Elements of customary international law.
  • The prohibition on the use of force as a jus cogens norm.
  • Customary international law status of the principle of non-intervention.

The competence of the ICJ to give its determination based on customary international law

1. The United States when accepting the compulsory jurisdiction of the ICJ (under Article 36(2) of the ICJ Statute) entered into the Vandenberg reservation. This reservation barred the ICJ from using certain multilateral treaties in the adjudication of the dispute.

2. The United States held that this reservation barred the Court from determining the case even on the basis of customary and general principles of international law because customary law provisions, on which Nicaragua relied on, were identical to provisions in treaties sought to be excluded. Because of the identical content, the United States argued, treaty provisions supervene and subsume the parallel customary law provision (see below).

3. The Court disagreed. It held that multilateral treaty reservations could not preclude the Court from determining cases relying customary international law because the latter exists independently of treaty law.

NB: The United States disagreed with the Court’s determination to proceed with the case and refused to participate further, including at the merits stage (see the declaration made by the United States in this regard). Although the Court was barred from resorting to multilateral treaties, it referred to the latter, including the UN Charter, to identify the existence, nature and scope of various customary law principles. Commentators criticised the Court for circumventing the multilateral reservation in this manner.

Relationship between treaty law and customary international law

4. As we noted before, the United States argued that when customary international law and treaty law contain the same content; the treaty law subsumes and supervenes customary international law. In other words, “the existence of principles in the United Nations Charter precludes the possibility that similar rules might exist independently in customary international law, either because existing customary rules had been incorporated into the Charter, or because the Charter influenced the later adoption of customary rules with a corresponding content” (para 174).

5. In its response, the Court distinguished two situations:

(a)    Situations where the customary law principles were identical to treaty provisions; and

(b)    Situations where customary law and treaty law rights and obligations differed in respect of the same subject matter.

6. In situations where customary law principles were identical to treaty provisions (reflected as (a) above), the Court, quite correctly, disagreed with the view of the United States. It held that even if principles of customary international law are codified into treaties, the former continues to exist side by side with the latter. For treaty parties, both customary and treaty law apply and if, for some reason, the treaty ceases to apply the identical customary law provision continues to apply between them unaffected (see more on para 178).

7. The fact that customary international law exists alongside treaty law was an argument brought by Norway and Denmark in the North Sea Continental Shelf Cases. In these cases, the two countries having failed to attribute an obligation under Article 6 of the Geneva Conventions of 1958 to Germany, sought to bind Germany via customary international law. In this case the Court determined that Article 6 neither reflected customary law at the time of the codification, nor had it attained that status at the time of the determination. In the Nicaragua case, the Court relied on the North Sea Continental Shelf Cases to support the assertion that principles of customary international law can exist side by side with identical treaty law provisions and the latter does not supervene the former in a manner where the former ceases to exist (para 177).

8. The Court also relied on Article 51 of the UN Charter to show that a treaty itself can recognise the existence of customary international law on the same subject matter. The term “inherent” in Article 51 recognised that customary law rights of self-defense existed alongside treaty provisions.

9. Rules containing the same content could be treated differently in customary international law and in treaty law. For example, treaty law may contain institutions or mechanisms to ensure the effective implementation of its provisions, including those that reflect customary law. One could take the Court’s reading of Article 51 as an example. A State that exercises the right of self-defence under Article 51, according to the UN Charter, has an obligation to report the use of force immediately to the Security Council. The Court held that this was a treaty requirement and one that did not exist under customary law. Interestingly, although the failure to report did not result in a breach of customary international law, the Court indicated that the United State’s failure to observe this requirement contradicted her claim to be acting in self defence (see paras 200, 235).

10. The Court discussed situations where customary international law and treaty law provisions were not identical (see point (b) above). For example, the Court referred to the fact that concepts such and necessity and proportionality, or the definition of what constitutes an armed attack, are not found under Article 51, or the UN Charter, but in customary law. The Court concluded that (1) this proves that customary international law continues to exist alongside treaty law and that (2) areas governed by the two sources of law do not (always) overlap and the rules do not (always) have the same content.

“…the Charter, having itself recognized the existence of this right (inherent customary law right of self-defence under A. 51 of the UN Charter), does not go on to regulate directly all aspects of its content. For example, it does not contain any specific rule whereby self-defence would warrant only measures which are proportional to the armed attack and necessary to respond to it, a rule well established in customary international law. Moreover, a definition of the “armed attack” which, if found to exist, authorises the exercise of the “inherent right” of self-defence, is not provided in the Charter, and is not part of treaty law. It cannot therefore be held that Article 51 is a provision which “subsumes and supervenes” customary international law.”

11. In case of a divergence between treaty law and customary international law, for the parties to the treaty, amongst themselves, the treaty provisions apply as lex specialis. The court’s support for this principle can be found in paras 180 and 181. The Court, in conclusion, explained the relationship between the UN Charter and customary international law in the following manner:

“However, so far from having constituted a marked departure from a customary international law which still exists unmodified, the Charter gave expression in this field (on the use of force and self defence) to principles already present in customary international law, and that law has in the subsequent four decades developed under the influence of the Charter, to such an extent that a number of rules contained in the Charter have acquired a status independent of it. The essential consideration is that both the Charter and the customary international law flow from a common fundamental principle outlawing the use of force in international relations. The differences which may exist between the specific content of each are not, in the Court’s view, such as to cause a judgment confined to the field of customary international law to be ineffective or inappropriate (to the parties of the Charter who are bound by the Charter)… (text in brackets added)(para 181).”

The relationship between customary international law and jus cogens

13. The court cited material presented by Nicaragua, the United States and the International Law Commission to argue that the prohibition on the use of force contained in Article 2(4) of the UN Charter has attained the status of a jus cogens norm.  The Court found this to be “A further confirmation of the validity as customary international law of the principle of the prohibition of the use of force expressed in Article 2, paragraph 4, of the Charter of the United Nations…” (para 190).

The necessary elements to determine the existence of customary international law

14. The Court, similar to the North Sea Continental Shelf Case, considered both the subjective element (opinio juris) and the objective element (State practice) as essential pre-requisites to the formation and elucidation of a customary international law norm (para 207). The jurisprudence of the Nicaragua case contained an important clarification – inconsistent State practice does not affect the formation or continued existence of a customary principle so long as the inconsistency is justified as a breach of the rule.

“It is not to be expected that in the practice of States the application of the rules in question should have been perfect, in the sense that States should have refrained, with complete consistency, from the use of force or from intervention in each other’s internal affairs.  

The Court does not consider that, for a rule to be established as customary, the corresponding practice must be in absolutely rigorous conformity with the rule.   In order to deduce the existence of customary rules, the Court deems it sufficient that the conduct of States should, in general, be consistent with such rules, and that instances of State conduct inconsistent with a given rule should generally have been treated as breaches of that rule, not as indications of the recognition of a new rule.  

If a State acts in a way prima facie incompatible with a recognized rule, but defends its conduct by appealing to exceptions or justifications contained within the rule itself, then whether or not the State’s conduct is in fact justifiable on that basis, the significance of that attitude is to confirm rather than to weaken the rule. (para 186)”  

15. The Nicaragua jurisprudence explained how one could deduct opinio juris from acts of State. The Court held that opinio juris could be deduced from:

–   the attitude of States towards certain General Assembly resolutions. For example, the “Declaration on Principles of International Law concerning Friendly Relations…” (hereafter called the Declaration on Friendly Relations). The Court held that:

“The effect of consent to the text of such resolutions cannot be understood as merely that of a “reiteration or elucidation” of the treaty commitment undertaken in the Charter. On the contrary, it may be understood as an acceptance of the validity of the rule or set of rules declared by the resolution by themselves…It would therefore seem apparent that the attitude referred to expresses an opinio juris respecting such rule (or set of rules), to be thenceforth treated separately from the provisions, especially those of an institutional kind, to which it is subject on the treaty-law plane of the Charter” 

–   Statements by State representatives.

–   Obligations undertaken by participating States in international forums (the Court provided the example of the Conference on Security and Co-operation in Europe, Helsinki)

–   The International Law Commission’s findings that a concept amounts to a customary law principle.

–   Multilateral conventions.

NB: The fact that the Court relied on resolutions of the United Nations to deduct opinio juris was subject to criticism. As you know, opinio juris is the subjective element necessary to form customary law. Opinio juris is reflected in instances where the State undertakes a particular practice because it believes that it is legally bound to do so. Voting patterns in the United Nations are often guided by policy considerations over legal merits. The General Assembly’s subject matter is more policy oriented than legal (for which we have the 6th Committee). For example, when the United States voted for the Friendly Relations Declaration it stated on record its belief that the Declaration was “only a statement of political intention and not an expression of the law.”  This is not to say that provisions on General Assembly Resolutions that guide  the international community to act in a certain way may not eventually become binding international law (either by attaining customary law status or becoming codified into treaty law). It can, if there is adequate State practice and opinio juris. The argument is that opinio juris cannot be said to exist based merely on a vote in favour of a non-binding resolution – in the absence of an examination of subsequent consistent and general State practice (which, in turn, reflects or confirms opinio juris).  

Customary international law relating to principles of non-intervention

16. The Court held that “Principles such as those of the non-use of force (para 191), non-intervention (para 192), respect for the independence and territorial integrity of States, right of collective self defence (para 193) and the freedom of navigation, continue to be binding as part of customary international law, despite the operation of provisions of conventional law in which they have been incorporated (text in brackets added).”

17.  The Court’s finding that principle of non-intervention formed a part of customary international law invited criticism from commentators, partly because they disagreed that the principle formed customary international law and partly because of the Court’s own contradictions in coming to its conclusions and inadequacy of analysis (see below). The Court’s contradiction stems from this statement: ” The principle of non-intervention involves the right of every sovereign State to conduct its affairs without outside interference; though examples of trespass against this principle are not infrequent, the Court considers that it is part and parcel of customary international law…”(emphasis added. Para 202).

18.  The Court began its analysis with two questions: “Notwithstanding the multiplicity of declarations by States accepting the principle of non-intervention, there remain two questions: first, what is the exact content of the principle so accepted, and secondly, is the practice sufficiently in conformity with it for this to be a rule of customary international law?” The first question was discussed in a previous post and will not be discussed here.

18. Although the question seemed to direct the Court towards identifying an existing custom, in its response the Court seemed to have already determined that the customary law prohibition of non-intervention existed. In the following passage the Court deliberates if, in contrast, a customary law right to intervention had evolved.

“There have been in recent years a number of instances of foreign intervention for the benefit of forces opposed to the government of another State. The Court is not here concerned with the process of decolonisation… It has to consider whether there might be indications of a practice illustrative of belief in a kind of general right for States to intervene, directly or indirectly, with or without armed force, in support of an internal opposition in another State, whose cause appeared particularly worthy by reason of the political and moral values with which it was identified. For such a general right to come into existence would involve a fundamental modification of the customary law principle of non-intervention.” (paras 206, 207). 

19. The Court went on to hold, as before, that for a new customary rule to be formed, not only must the acts concerned “amount to a settled practice”, but they must be accompanied by the opinio juris sive necessitates”.

“The significance for the Court of cases of State conduct prima facie inconsistent with the principle of non-intervention lies in the nature of the ground offered as justification. Reliance by a State on a novel right or an unprecedented exception to the principle might, if shared in principle by other States, tend towards a modification of customary international law. In fact however the Court finds that States have not justified their conduct by reference to a new right of intervention or a new exception to the principle of its prohibition. The United States authorities have on some occasions clearly stated their grounds for intervening in the affairs of a foreign State for reasons connected with, for example, the domestic policies of that country, its ideology, the level of its armaments, or the direction of its foreign policy. But these were statements of international policy, and not an assertion of rules of existing international law.”

20.  The Court also noted that the United States has not sought to justify its intervention in Nicaragua on legal grounds, but had only justified it at a political level.  The United States had not asserted for itself  legal right of intervention in these circumstances. The Court, without further analysis into State practice, almost immediately proceeded to find that “…no such general right of intervention, in support of an opposition within another State, exists in contemporary international law. The Court concludes that acts constituting a breach of the customary principle of non-intervention will also, if they directly or indirectly involve the use of force, constitute a breach of the principle of non-use of force in international relations (para 209).”

Development of a parallel customary international law?

In addition to the comments made above in italics, another interesting aspect of the judgment is that it sought to divorce customary international law obligation from the identical treaty obligation because of the jurisdictional bar to consider multilateral treaties. In its consideration of customary international law it developed certain principles independently of the treaty. For example, Article 2(4) of the UN Charter prohibits the threat or use of force against another State. The Court held that the same prohibition on the use of force could be found under customary international law and as a jus cogens norm. The Court then went on to categorize the use of force under customary law as either a “grave use of force” (i.e. use of force amounting to an armed attack) or a “less grave use of force” (i.e. use of force that falls short of an armed attack – for example, the threat to use force). The Court, then, restricted the right of self-defense to a situation where there had been a grave use of force (or an armed attack, as defined by the Court). If one were to hold that the relevant Charter principles were clear, precise and unambiguous, one could say this divorced interpretation could result in customary law developing in a manner that is not in line with the Charter and thereby creating separate rights/ regimes of law that govern the same subject matter. This is because, then, the two regimes would be irreconcilable. However, the fact remains that the Charter does leave room for interpretation – for example, on the definition of an armed attack or on the use of force. In cases of ambiguity, Article 31 of the Vienna Convention on the Law of Treaties directs us to look at, inter alia, subsequent practice and any relevant rules of international law that maybe applicable. In other words, a treaty can be interpreted with the assistance of customary and general principles of international law. In this case, the development of customary law would also mean a potential development of ambiguous treaty law – and a reconciliation of treaty and customary law provisions.

Material on the Nicaragua case

The following contains a list of scholarly articles and other material that discuss the Nicaragua case. If you would like to add to the list, please note your suggestions in the comment box. 

The judgment including separate opinions of individual judges and summaries of the judgment and orders

The World Court and Jus Cogens, 81 AMJIL 93, Gorden A. Christenson. Christenson argues that  an independent development of the customary law right divorced from the treaty can have wider consequences:

We have then a double irony. The Court uses the United States position accepting the treaty norm against the threat or use of force also as a customary norm possibly having jus cogens quality, in part, to justify taking jurisdiction as a matter quite independent of the norm that otherwise falls under the multilateral treaty reservation. Since there are two separate sources of the law, the choice of the one source rather than the other means that the norm relied upon survives the jurisdictional bar to the use of the other. Yet the two norms are not different enough to undermine completely the content of the Charter norm. This formalism simply masks the more interesting question of the Court’s institutional claim, given the ineffectiveness of the UN Security system, to develop an international public order case by case, by breaking away form the strictures of the Charter and treaty norms. The Court untied the treaty norms from their constraints within the United Nations or regional collective security systems, a potentially destabilizing decision, one whose consequences are unforeseen.  The decision based on the validity of an autonomous norm of customary international law free from the Charter is a constitutive one of potential great significance (81 AMJIL 100, 1987).

Trashing customary international law, Antony D’Amato, 81 AMJIL 102 (1987) (full text): (D’Amato discusses the paucity of State practice examined by the international court of justice before concluding that the principle non-intervention formed part of customary international law. He argues that the acceptance of General Assembly resolutions do not manifest opinio juris. He states that the Court failed to consider that Article 2(4) continued to evolve through the years.)

The World Court’s Achievement, Richard Falk, 81 AMJIL 106 (Falk takes a generally positive approach to the judgment, gives a good overview of the case and Judge Shwebel’s dissent)

Drawing the right line, Tom J. Farer, 81 AMJIL 112 (Farer takes a cold-war contextual approach to the judgment and supports the Court’s narrow view of an armed attack and self defence).

Some observations on the ICJ’s procedural and substantive innovations, Thomas M. Franck, 81 AMJIL 116 (criticizes the determination of relevant State practice in relation to non-intervention and the reliance on UN resolutions to illicit opinio juris (it alleges that the Court sought to harden soft law prematurely). Frank points out that the interventions falling short of armed attacks would not allow States to target rebel groups in another State’s territory even if the insurgency is planned, trained, armed and directed from that territory).

Protecting the Court’s institutional interests: Why not the Marbury approach? Michael J. Glennon, 81 AMJIL 121 (discusses reservations before the ICJ and the Court’s prerogative to determine its own jurisdiction)

Discretion to decline to exercise jurisdiction, Edward Gorden, 81 AMJIL 129 (discusses the discretionary power of the court to decline to exercise its jurisdiction at the merit stages).

The Nicaragua judgment and the future of the law of force and self-defense, John Lawrence Hargrove 81AMJIL 135 (Hargrove criticizes the ICJ’s construction of the notion of collective self defense, armed attack and forcible countermeasures).

Somber reflections on the compulsory jurisdiction of the international court, Mark Weston Janis, 81 AMJIL 144

Custom on a sliding scale, Frederic L. Kirgis 81 AMJIL 146 (Kirgis discusses the relationship between State practice and opinio juris, criticizes the methods (or lack thereof) of the Court in determining the customary law nature of Article 2(4) of the Charter. Points out that actual State practice on intervention did not support the Court’s findings).

The International Court lives unto its name, Herbert W. Briggs, 81 AMJIL 78.

Determining US responsibility for contra operations under international law, Francis V. Boyle

Customary international law in the Nicaragua Case, Rijpkema. (Abstract: On 27 June 1986 the International Court of Justice passed judgment in the case concerning military and paramilitary activities in and against Nicaragua. Because of a reservation that the United States had made when it accepted the jurisdiction of the Court, the Court could not pronounce a decision regarding the dispute insofar as it concerned multilateral convertions. As a resuld of this, the Court was compelled to base its judgment largely on rules of customary international law and general principles of law. The rules of customary law which were relevant for the judgment corresponded to a significant extent, as regards their content, to the rules of treaty law which the Court was unable to apply, such as the prohibition on the use of force of Article 2, paragraph 4 of the Charter of the United Nations. This led the Court to indicate in precise terms how rules of treaty law and rules of customary law which have a corressponding content can co-exist and how the existence of rules of customary international law can be established in general. In addition, the Court examined in some detail the existence and content of certain specific rules of customary international law.)

Le peuple, c’est moi!The world court and human rights, 81 AMJIL 173 

LJIL Symposium: Discussion of the ICJ Nicaragua Judgment

The Impact of the Nicaragua Case on the Court and Its Role: Harmful, Helpful, or In Between?, Lori Fisler Damrosch (Abstract: At the time the United States withdrew from participation in the Nicaragua case at the International Court of Justice, the US government expressed concern that ‘the course on which the Court may now be embarked could do enormous harm to it as an institution and to the cause of international law’. This essay examines whether or to what extent the anticipated negative effects came to pass. It concludes that dire predictions of harm to the Court were overstated. Twenty-five years later, the rate at which states accept the Court’s jurisdiction has held steady. Only a few states have added jurisdictional reservations concerning military activities. The mix of cases being brought to the Court has shifted towards a more representative distribution. States are generally complying with the Court’s decisions, though some compliance problems remain. The most serious negative impact has been on the willingness of the United States (still the Court’s most active litigant) to participate fully in international dispute settlement.)

LJIL Symposium: The Nicaragua Case: Its Impact, John Dugard

LJIL Symposium: Response of Lori F. Damrosch to Comments by John Dugard, Lori F. Damrosch

The Principle of Non-Intervention 25 Years after the Nicaragua Judgment, by Marcelo Kohen (Abstract: This article focuses on the analysis by the International Court of Justice of the principle of non-intervention in domestic affairs in its judgment of 27 June 1986 in the case concerning Military and Paramilitary Activities in and against Nicaragua and contrasts it with the evolution of international law and practice in this field. It is proposed that the Court’s 1986 analysis not only remains of actuality today, but also constitutes a precursor to legal developments that have since taken place. This is particularly the case with regard to the relationship between the protection of human rights on the one hand and the safeguard of state sovereignty and the collective security regime on the other. The 1986 judgment helped to clarify the content of humanitarian assistance. It constituted the starting point for the development of this concept in a series of GA resolutions that were subsequently adopted. The controversial doctrine of ‘humanitarian intervention’, as well as state practice in violation of this principle, in no way led to modifying existing international law. Similarly, the new concept of ‘responsibility to protect’, which places emphasis on collective security and discounts unilateral action, has not led to the disappearance of the principle of non-intervention either.)

LJIL Symposium: From Nicaragua to R2P: Continuity and Change, André Nollkaemper

© Ruwanthika Gunaratne and Public International Law at, 2008 – present. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Ruwanthika Gunaratne and Public International Law with appropriate and specific direction to the original content.

ILC issues first report on customary international law


During its sixty-fourth session, in 2012, the International Law Commission (ILC) decided to place the topic “Formation and evidence of customary international law” on its current programme of work, and appointed Michael Wood as Special Rapporteur. In his first report, Michael Wood outlined his proposed contribution. He discussed  earlier work of the ILC on the topic and identified the aim of the report, inter-alia, as a source of “…guidance to those called upon to apply rules of customary international law on how to identify such rules in concrete cases” and to describe the current state of international law on the formation and evidence of rules of customary international law, without prejudice to developments that might occur in the future. He discussed succinctly customary international law’s relationship to other sources listed in Article 38 of the Statute of the International Court of Justice, including treaties and general principles of international law. An interesting aspect is that he disputes arguments that treaties may be “a more important source of international law” than custom. He argues that “such generalizations are neither particularly illuminating nor necessarily accurate.” He argues that,

“Even in fields where there are widely accepted “codification” conventions, the rules of customary international law continue to govern questions not regulated by the conventions and continue to apply in relations with and between non-parties. Rules of customary international law may also fill possible lacunae in treaties, and assist in their interpretation. An international court may also decide that it may apply customary international law where a particular treaty cannot be applied because of limits on its jurisdiction (for example in the Nicaragua case).”

While the above mentioned aspects, including various terminology used to describe customary international law, its constituent elements and attitude of States would be expanded in successive reports,  Wood gives an overview of various jurisprudence of the ICJ and other international, regional and national courts and tribunals on the formation of customary international law. Since we look at some of the ICJ’s jurisprudence in our class, I  extracted the section(paras 54 – 65) on the ICJ’s practice in deducing customary law below (minus the footnotes, which are also informative).

The full report can be accessed here. Students are strongly encouraged to read pp 21 -41.


54. The case law of the International Court of Justice and its predecessor, the Permanent Court of International Justice, will be of great significance for the Commission’s work on the present topic. The Court’s primary function in relation to customary international law is to identify and apply customary rules as necessary for deciding the cases before it.Its judgments (including separate and dissenting opinions) shed much light on the general approach to the formation and evidence of customary international law (when “[w]hat ‘is’ becomes what ‘must be’”), including on specific aspects of these processes.

55. Examining the Court’s frequent application of Article 38.1 (b) of its Statute, by which it “perform[s] its perfectly normal function of assessing the various elements of State practice and legal opinion adduced … as indicating the development of a rule of customary law”, affords an overview of the Court’s approach to the matter. As the judgments referred to below indicate, the Court has clearly and consistently held — as did its predecessor — that customary international law is formed through State practice accompanied by opinio juris.

56. In the Lotus case, the Permanent Court of International Justice stated that international law emanates from the free will of States as expressed in conventions or “by usages generally accepted as expressing principles of law”. It emphasized the distinction between the two constitutive elements of customary international law, stressing the need for both to be present in order to ground a finding of such law:

“Even if the rarity of the judicial decisions to be found among the reported cases were sufficient to prove in point of fact the circumstances … it would merely show that States had often, in practice, abstained from instituting criminal proceedings, and not that they recognised themselves as being obliged to do so; for only if such abstentions were based on their being conscious of having a duty to abstain, would it be possible to speak of an international custom.”

57. The classic statement of the International Court of Justice on the processes of formation and evidence of rules of customary international law is to be found in the North Sea Continental Shelf cases:

“Although the passage of only a short period of time is not necessarily, or of itself, a bar to the formation of a new rule of customary international law on the basis of what was originally a purely conventional rule, an indispensable requirement would be that within the period in question, short though it might be, State practice, including that of States whose interests are specially affected, should have been both extensive and virtually uniform in the sense of the provision invoked; and should moreover have occurred in such a way as to show a general recognition that a rule of law or legal obligation is involved.

… The essential point in this connection — and it seems necessary to stress it — is that even if these instances of action by non-parties to the Convention were much more numerous than they in fact are, they would not, even in the aggregate, suffice in themselves to constitute the opinio juris; — for in order to achieve this result, two conditions must be fulfilled. Not only must the acts concerned amount to a settled practice [une pratique constante, in the French text], but they must also be such or be carried out in such a way as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it. The need for such a belief i.e. the existence of a subjective element, is implicit in the very notion of the opinio juris sive necessitatis. The States concerned must therefore feel that they are conforming to what amounts to a legal obligation. The frequency, or even habitual character of the acts is not in itself enough. There are many international acts, e.g. in the field of ceremonial and protocol, which are performed almost invariably, but which are motivated only by considerations of courtesy, convenience or tradition, and not by any legal sense of duty.”

58. The Court reaffirmed this in Military and Paramilitary Activities in and against Nicaragua, where it said that in order to consider what rules of customary international law were applicable it “has to direct its attention to the practice and opinio juris of States”, and that:

“… as was observed in the North Sea Continental Shelf cases, for a new customary rule to be formed, not only must the acts concerned ‘amount to a settled practice’ but they must be accompanied by the opinio juris sive necessitatis. Either the States taking such action or other States in a position to react to it, must have behaved so that their conduct is ‘evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it. The need for such a belief, i.e. the existence of a subjective element, is implicit in the very notion of opinio juris sive necessitatis’.”

59. In its judgment in Continental Shelf (Tunisia/Libyan Arab Jamahiriya) the Court referred to “the actual practice of States” as “expressive, or creative, of customary rules”. In the Gulf of Maine case, a Chamber of the Court observed that customary international law “comprises a limited set of norms for ensuring the co-existence and vital co-operation of the members of the international community, together with a set of customary rules whose presence in the opinio juris of States can be tested by induction based on the analysis of a sufficiently extensive and convincing practice, and not by deduction from preconceived ideas”.

60. When turning to an examination of customary international law in its Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons, the Court said at the outset that “[a]s the Court has stated, the substance of that law must be ‘looked for in the actual practice and opinio juris of States’ (Continental Shelf (Libyan Arab Jamahiriya/Malta), Judgment, I.C.J. Reports 1985, pp. 29-30, para. 27)”. Later in the Opinion it noted the existence of customary rules that “have been developed by the practice of States”.

61. The most recent extended pronouncement of the Court on its basic approach is to be found in Germany v. Italy, in which it said:

“It follows that the Court must determine, in accordance with Article 38 (1) (b) of its Statute, the existence of ‘international custom, as evidence of a general practice accepted as law’ … To do so, it must apply the criteria which it has repeatedly laid down for identifying a rule of customary international law. In particular, as the Court made clear in the North Sea Continental Shelf cases, the existence of a rule of customary international law requires that there be ‘a settled practice’ together with opinio juris …

Moreover, as the Court has also observed,

“It is of course axiomatic that the material of customary international law is to be looked for primarily in the actual practice and opinio juris of States, even though multilateral conventions may have an important role to play in recording and defining rules deriving from custom, or indeed in developing them. (Continental Shelf (Libyan Arab Jamahiriya/Malta), Judgment, I.C.J. Reports 1985, pp. 29-30, para. 27).”

62. At the risk of oversimplification, it may be said that there are two main approaches to the identification of particular rules of customary international law in the case law of the Court. In some cases the Court finds that a rule of customary international law exists (or does not exist) without detailed analysis.This may be because the matter is considered obvious (for example, because it is based on a previous finding of the Court or on what the Court views as unquestioned law). A number of examples may be found in the Court’s judgment of 19 November 2012 in Nicaragua v. Colombia (see footnote). In other cases the Court engages in a more detailed analysis of State practice and opinio juris in order to determine the existence or otherwise of a rule of customary international law. The Court’s judgment of 3 February 2012 in Germany v. Italy illustrates this approach. It is particularly these latter cases that are helpful in illustrating the Court’s approach to the formation and evidence of customary international law.

63. There is a considerable number of cases in which the Court has addressed specific aspects of the process of formation and identification of rules of customary international law, covering many of the issues that arise under the present topic, chief among them the nature of the State practice and opinio juris elements, and the relationship between treaties and customary international law. While such cases do not provide complete answers, they offer valuable guidance…

64. It is widely recognized in the literature that the International Court, through its jurisprudence, has enhanced the role of customary international law and clarified some of its aspects. At the same time, commentators have suggested that the Court has thus far provided only limited guidance on how a rule of customary international law is formed and is to be ascertained, having “a marked tendency to assert the existence of a customary rule more than to prove it”, and ultimately following a rather flexible approach. It has moreover been observed that the Court has not always been consistent in its use of terminology relating to customary international law, or in distinguishing the latter from general principles of law.

65. The President of the International Court of Justice, addressing the issue of the Court’s approach to customary international law, has recently explained:

“… authors are correct in drawing attention to the prevalent use of general statements of rules in the Court’s modern practice, although they take the point too far by insisting on theorizing this development. In fact, the Court has never abandoned its view, firmly rooted in the wording of the Statute, that customary international law is ‘general practice accepted as law’ — that is, in the words of a recent case, that ‘the existence of a rule of customary international law requires that there be a ‘settled practice’ together with opinio juris’. However, in practice, the Court has never found it necessary to undertake such an inquiry for every rule claimed to be customary in a particular case and instead has made use of the best and most expedient evidence available to determine whether a customary rule of this sort exists. Sometimes this entails a direct review of the material elements of custom on their own, while more often it will be sufficient to look to the considered views expressed by States and bodies like the International Law Commission as to whether a rule of customary law exists and what its content is, or at least to use rules that are clearly formulated in a written expression as a focal point to frame and guide an inquiry into the material elements of custom.” 

Read the full report here. For subsequent work of the Commission on this subject, click here.


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