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World Court Digest

I. Substantive International Law - First Part
2.2. Customary International Law
2.2.1. Formation of Customary International Law

¤ Military and Paramilitary Activities
(Nicaragua/United States of America)
Merits. J. 27.6.1986
I.C.J. Reports 1986, p. 14

[pp. 97-98] In view of this conclusion, the Court has next to consider what are the rules of customary international law applicable to the present dispute. For this purpose, it has to direct its attention to the practice and opinio juris of States; as the Court recently 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), I.C.J. Reports 1985, pp. 29-30, para. 27.)

In this respect the Court must not lose sight of the Charter of the United Nations and that of the Organization of American States, notwithstanding the operation of the multilateral treaty reservation. Although the Court has no jurisdiction to determine whether the conduct of the United States constitutes a breach of those conventions, it can and must take them into account in ascertaining the content of the customary international law which the United States is also alleged to have infringed.
The Court notes that there is in fact evidence, to be examined below, of a considerable degree of agreement between the Parties as to the content of the customary international law relating to the non-use of force and non-intervention. This concurrence of their views does not however dispense the Court from having itself to ascertain what rules of customary international law are applicable. The mere fact that States declare their recognition of certain rules is not sufficient for the Court to consider these as being part of customary international law, and as applicable as such to those States. Bound as it is by Article 38 of its Statute to apply, inter alia, international custom "as evidence of a general practice accepted as law", the Court may not disregard the essential role played by general practice. Where two States agree to incorporate a particular rule in a treaty, their agreement suffices to make that rule a legal one, binding upon them; but in the field of customary international law, the shared view of the Parties as to the content of what they regard as the rule is not enough. The Court must satisfy itself that the existence of the rule in the opinio juris of States is confirmed by practice ...
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.

[pp. 99-101] The Court thus finds that both Parties take the view that the principles as to the use of force incorporated in the United Nations Charter correspond, in essentials, to those found in customary international law. The Parties thus both take the view that the fundamental principle in this area is expressed in the terms employed in Article 2, paragraph 4, of the United Nations Charter. They therefore accept a treaty-law obligation to refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the purposes of the United Nations. The Court has however to be satisfied that there exists in customary international law an opinio juris as to the binding character of such abstention. This opinio juris may, though with all due caution, be deduced from, inter alia, the attitude of the Parties and the attitude of States towards certain General Assembly resolutions, and particularly resolution 2625 (XXV) entitled "Declaration on Principles of International Law concerning Friendly Relations and Co-operation among States in accordance with the Charter of the United Nations". 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. The principle of non-use of force, for example, may thus be regarded as a principle of customary international law, not as such conditioned by provisions relating to collective security, or to the facilities or armed contingents to be provided under Article 43 of the Charter. 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.
As regards the United States in particular, the weight of an expression of opinio juris can similarly be attached to its support of the resolution of the Sixth International Conference of American States condemning aggression (18 February 1928) and ratification of the Montevideo Convention on Rights and Duties of States (26 December 1933), Article 11 of which imposes the obligation not to recognize territorial acquisitions or special advantages which have been obtained by force. Also significant is United States acceptance of the principle of the prohibition of the use of force which is contained in the declaration on principles governing the mutual relations of States participating in the Conference on Security and Co-operation in Europe (Helsinki, 1 August 1975), whereby the participating States undertake to "refrain in their mutual relations, as well as in their international relations in general," (emphasis added) from the threat or use of force. Acceptance of a text in these terms confirms the existence of an opinio juris of the participating States prohibiting the use of force in international relations.
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 may be found in the fact that it is frequently referred to in statements by State representatives as being not only a principle of customary international law but also a fundamental or cardinal principle of such law. The International Law Commission, in the course of its work on the codification of the law of treaties, expressed the view that "the law of the Charter concerning the prohibition of the use of force in itself constitutes a conspicuous example of a rule in international law having the character of jus cogens" (paragraph (1) of the commentary of the Commission to Article 50 of its draft Articles on the Law of Treaties, ILC Yearbook, 1966-II, p. 247). Nicaragua in its Memorial on the Merits submitted in the present case states that the principle prohibiting the use of force embodied in Article 2, paragraph 4, of the Charter of the United Nations "has come to be recognized as jus cogens". The United States, in its Counter-Memorial on the questions of jurisdiction and admissibility, found it material to quote the views of scholars that this principle is a "universal norm", a "universal international law", a "universally recognized principle of international law", and a "principle of jus cogens".
As regards certain particular aspects of the principle in question, it will be necessary to distinguish the most grave forms of the use of force (those constituting an armed attack) from other less grave forms. In determining the legal rule which applies to these latter forms, the Court can again draw on the formulations contained in the Declaration on Principles of International Law concerning Friendly Relations and Co-operation among States in accordance with the Charter of the United Nations (General Assembly resolution 2625 (XXV), referred to above). As already observed, the adoption by States of this text affords an indication of their opinio juris as to customary international law on the question. Alongside certain descriptions which may refer to aggression, this text includes others which refer only to less grave forms of the use of force.

[pp. 106-109] 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. As the Court has observed: "Between independent States, respect for territorial sovereignty is an essential foundation of international relations" (I.C.J. Reports 1949, p. 35), and international law requires political integrity also to be respected. Expressions of an opinio juris regarding the existence of the principle of non-intervention in customary international law are numerous and not difficult to find. Of course, statements whereby States avow their recognition of the principles of international law set forth in the United Nations Charter cannot strictly be interpreted as applying to the principle of non-intervention by States in the internal and external affairs of other States, since this principle is not, as such, spelt out in the Charter. But it was never intended that the Charter should embody written confirmation of every essential principle of international law in force. The existence in the opinio juris of States of the principle of non-intervention is backed by established and substantial practice. It has moreover been presented as a corollary of the principle of the sovereign equality of States. A particular instance of this is General Assembly resolution 2625 (XXV), the Declaration on the Principles of International Law concerning Friendly Relations and Co-operation among States. In the Corfu Channel case, when a State claimed a right of intervention in order to secure evidence in the territory of another State for submission to an international tribunal (I.C.J. Reports 1949, p. 34), the Court observed that:

"the alleged right of intervention as the manifestation of a policy of force, such as has, in the past, given rise to most serious abuses and such as cannot, whatever be the present defects in international organization, find a place in international law. Intervention is perhaps still less admissible in the particular form it would take here; for, from the nature of things, it would be reserved for the most powerful States, and might easily lead to perverting the administration of international justice itself." (I.C.J. Reports 1949, p. 35.)

The principle has since been reflected in numerous declarations adopted by international organizations and conferences in which the United States and Nicaragua have participated, e.g., General Assembly resolution 2131 (XX), the Declaration on the Inadmissibility of Intervention in the Domestic Affairs of States and the Protection of their Independence and Sovereignty. It is true that the United States, while it voted in favour of General Assembly resolution 2131 (XX), also declared at the time of its adoption in the First Committee that it considered the declaration in that resolution to be "only a statement of political intention and not a formulation of law" (Official Records of the General Assembly, Twentieth Session, First Committee, A/C.1/SR. 1423, p. 436). However, the essentials of resolution 2131 (XX) are repeated in the Declaration approved by resolution 2625 (XXV), which set out principles which the General Assembly declared to the "basic principles" of international law, and on the adoption of which no analogous statement was made by the United States representative.
As regards inter-American relations, attention may be drawn to, for example, the United States reservation to the Montevideo Convention on Rights and Duties of States (26 December 1933), declaring the opposition of the United States Government to "interference with the freedom, the sovereignty or other internal affairs, or processes of the Governments of other nations"; or the ratification by the United States of the Additional Protocol relative to Non-Intervention (23 December 1936). Among more recent texts, mention may be made of resolutions AG/RES.78 and AG/RES. 128 of the General Assembly of the Organization of American States. In a different context, the United States expressly accepted the principles set forth in the declaration, to which reference has already been made, appearing in the Final Act of the Conference on Security and Co-operation in Europe (Helsinki, 1 August 1975), including an elaborate statement of the principle of non-intervention; while these principles were presented as applying to the mutual relations among the participating States, it can be inferred that the text testifies to the existence, and the acceptance by the United States, of a customary principle which has universal application.
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? As regards the first problem - that of the content of the principle of non-intervention - the Court will define only those aspects of the principle which appear to be relevant to the resolution of the dispute. In this respect it notes that, in view of the generally accepted formulations, the principle forbids all States or groups of States to intervene directly or indirectly in internal or external affairs of other States. A prohibited intervention must accordingly be one bearing on matters in which each State is permitted, by the principle of State sovereignty, to decide freely. One of these is the choice of a political, economic social, and cultural system, and the formulation of foreign policy. Intervention is wrongful when it uses methods of coercion in regard to such choices, which must remain free ones. The element of coercion, which defines, and indeed forms the very essence of, prohibited intervention, is particularly obvious in the case of an intervention which uses force, either in the direct form of military action, or in the indirect form of support for subversive or terrorist armed activities within another State. As noted above (paragraph 191), General Assembly resolution 2625 (XXV) equates assistance of this kind with the use of force by the assisting State when the acts committed in another State "involve a threat or use of force". These forms of action are therefore wrongful in the light of both the principle of non-use of force, and that of non-intervention. In view of the nature of Nicaragua's complaints against the United States, and those expressed by the United States in regard to Nicaragua's conduct towards El Salvador, it is primarily acts of intervention of this kind with which the Court is concerned in the present case.
However, before reaching a conclusion on the nature of prohibited intervention, the Court must be satisfied that State practice justifies it. 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 decolonization; this question is not in issue in the present case. 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.
In considering the instances of the conduct above described, the Court has to emphasize 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 the opinio juris sive necessitatis." (I.C.J. Reports 1969, p. 44, para. 77.)

The Court has no jurisdiction to rule upon the conformity with international law of any conduct of States not parties to the present dispute, or of conduct of the Parties unconnected with the dispute nor has it authority to ascribe to States legal views which they do not themselves advance. 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.

[pp. 110-111] Furthermore, the Court has to recall that the United States itself is relying on the "inherent right of self-defence" (paragraph 126 above), but apparently does not claim that any such right exists as would, in respect of intervention, operate in the same way as the right of collective self-defence in respect of an armed attack. In the discharge of its duty under Article 53 of the Statute, the Court has nevertheless had to consider whether such a right might exist ; but in doing so it may take note of the absence of any such claim by the United States as an indication of opinio juris.