Nicaragua vs United States: An Analysis of the Jurisprudence Relating to Customary International Law (2 of 2)

International Court of Justice Contentious Case: Case Concerning the Military and Paramilitary Activities In and Against Nicaragua (Nicaragua vs United States)
Year of Decision: 1986.

Click here for a summary of the Court’s deliberations on aspects relating to the use of force, non-intervention, and issues relating to sovereignty.


This case was about military and paramilitary activities conducted by, or with the assistance of, the United States against Nicaragua from 1981 to 1984.

Due to a United States’ multilateral treaty reservation, (the Vandenberg reservation), the Court could not rely on the United Nations Charter and was compelled to base its findings in relation to the use of force customary and general principles of international law. As a result, the Nicaragua case developed a significant jurisprudence on customary international law relating to (1) the use of force and non-intervention, (2) elements necessary to form customary international law, and (3) the relationship between customary and treaty law.

Controversial aspects of the decision included (1) the Court’s methodology used to determine that the principle of non-intervention had attained customary law status, (2) the Court’s reliance on resolutions of the General Assembly as a source of opinio juris and (3) the Court’s reliance on multilateral treaties to determine customary international law despite the Vandenberg reservation. 

Click here for Facts relating to the Case.

Questions before the Court:

In Nicaragua vs United States, the Court discussed, amongst others:

(1) Was the Court competent to give its determination based on customary international law when there was a multilateral treaty reservation?

(2) What is the relationship between treaty and customary international law?

(3) What are the elements necessary to form customary international law?

(4) What is the customary international law status of the principle of non-intervention?

Relevant findings of the Court:

(1) The Court held that multilateral treaty reservations cannot preclude the Court from relying on customary international law, because customary law exists independently of treaty law.  (paras 172 – 178)

The Court held that the issues raised by Nicaragua – relating the use of force and self defense – were regulated both by customary law and treaty law, in particular the Charter of the United Nations. Yet, the United States had entered into a multilateral treaty reservation, which did not, for example, allow the Court to rely on the Charter of the United Nations. The Court sought to rely, instead, exclusively on customary law relating to the use of force. In doing so, it held that multilateral treaty reservations cannot preclude the Court from relying on customary international law because, even if treaty provisions and customary law deal with the same subject matter, customary law exists independently of treaty law.

(2) Relationship between treaty and customary international law 

The Court examined the relationship in two contexts to demonstrate that customary and treaty law co-exist:

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

(b)   where there were different rights or obligations under customary and treaty law in respect of the same subject matter.

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

1. In situations where customary law principles were identical to treaty provisions, the Court held that even if principles of customary international law were subsequently codified into treaties, they continue to exist side by side. For parties to treaties, both customary and treaty law apply. If, for some reason, the treaty ceases to apply between treaty parties, the identical customary law provision continues to apply between them (para 178).

2. The argument that customary international law exists alongside treaty law was  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. The Court held that Article 6 did not reflect customary law at the time of the codification, and had not 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 its finding that principles of customary international law can exist side by side with identical treaty law provisions and that treaties do not supervene in a manner where the customary law ceases to exist (para 177).

3. 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 with respect to the same subject matter. For example, the Court said that the term “inherent” in Article 51 recognised that customary law rights of self-defense existed alongside treaty provisions.

4. Rules containing the same content could also 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 provisions that reflect existing customary law. For 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, while 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 defense (see paras 200, 235).

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

1. The Court discussed situations where customary international law and treaty law provisions were not identical. For example, the Court stated that concepts such as 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 Court held:

“…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.”

2. In case of a divergence between treaty law and customary international law, for the parties to a treaty, amongst themselves, the treaty provisions apply as lex specialis. (see paras 180 and 181). 

3. The Court explained the relationship between the Charter of the United Nations and customary international law on the use of force and self defense  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 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).”

4. The Court concluded that principles such as those of the non-use of force, non-intervention, respect for the independence and territorial integrity of States, right of collective self defense 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.” (paras 191-193).

Analysis: 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 separate 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. Then the two regimes may become 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.

(3) The Court held that opinio juris and State practice remain necessary elements to determine the existence of customary international law

1. In the Nicaragua case, as 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 norm (para 207).

2. On State practice, the jurisprudence of the Nicaragua case contained several important clarifications in respect of inconsistent State practice (para 186). The Court held that:

(a) For a customary rule to come into force, it is not necessary to have complete consistency in State practice in respect of the rule.

(b) Inconsistent State practice does not affect the formation or existence of a customary principle so long as the inconsistency is justified as a breach of the rule.

(c) This attempt at justifying a violation would only make the rule’s customary law nature stronger.

2. On opinio juris, the Nicaragua case jurisprudence elaborated on how to deduct opinio juris from acts of State. The Court held that the following reflected opinio juris:

(a) the attitude of States towards certain General Assembly resolutions.

“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” 

(b) Statements by State representatives.

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

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

(e) Multilateral conventions.

(4) The principle of non-intervention is customary international law. 

1. 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… 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.

3.  On State practice, the Court noted that even if “examples of trespass against this principle (of non-intervention) are not infrequent” (para 202), this did not affect the customary law nature of the prohibition on non-intervention. The Court held:

“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.”

4. On opinio juris, Court went on to hold, as before, that for a new customary rule to be formed, Sate practice must be accompanied by the opinio juris. The Court held:

“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… It (the Court) 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). 

5.  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).”

6. The Court held 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).

Analysis: General Assembly and Opinio Juris

Some commentators criticized the Court for its reliance on General Assembly resolutions to deduct opinio juris.They argued that voting patterns in the United Nations are often guided by policy and not by legal considerations. 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. 

Their 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). See also the Nuclear Weapons Advisory Opinion where the Court, once again, reflected on how General Assembly resolutions contribute towards the formation of customary law. 

 Background Reading Material on the Nicaragua case

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

Other reading material can be found in this page. It summarizes articles and other materials that maybe relevant to understand this case further.

© Ruwanthika Gunaratne  at, 2008 – 2020. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner, and without attribution to the author, is strictly prohibited. Excerpts and links may be used, without authorization, provided that full and clear credit is given to Ruwanthika Gunaratne  with appropriate and specific direction to the original content.

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