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World Court Digest

I. Substantive International Law - First Part
6.1. Use of Force

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

[pp.100-103] 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. In particular, according to this resolution:

"Every State has the duty to refrain from the threat or use of force to violate the existing international boundaries of another State or as a means of solving international disputes, including territorial disputes and problems concerning frontiers of States.


States have a duty to refrain from acts of reprisal involving the use of force.


Every State has the duty to refrain from any forcible action which deprives peoples referred to in the elaboration of the principle of equal rights and self-determination of that right to self-determination and freedom and independence.

Every State has the duty to refrain from organizing or encouraging the organization of irregular forces or armed bands, including mercenaries, for incursion into the territory of another State.

Every State has the duty to refrain from organizing, instigating, assisting or participating in acts of civil strife or terrorist acts in another State or acquiescing in organized activities within its territory directed towards the commission of such acts, when the acts referred to in the present paragraph involve a threat or use of force."

Moreover, in the part of this same resolution devoted to the principle of non-intervention in matters within the national jurisdiction of States, a very similar rule is found:

"Also, no State shall organize, assist, foment, finance, incite or tolerate subversive, terrorist or armed activities directed towards the violent overthrow of the regime of another State, or interfere in civil strife in another State."

In the context of the inter-American system, this approach can be traced back at least to 1928 (Convention on the Rights and Duties of States in the Event of Civil Strife, Art. I (1)); it was confirmed by resolution 78 adopted by the General Assembly of the Organization of American States on 21 April 1972. The operative part of this resolution reads as follows:

"The General Assembly Resolves:

1. To reiterate solemnly the need for the member states of the Organization to observe strictly the principles of nonintervention and self-determination of peoples as a means of ensuring peaceful coexistence among them and to refrain from committing any direct or indirect act that might constitute a violation of those principles.
2. To reaffirm the obligation of those states to refrain from applying economic, political, or any other type of measures to coerce another state and obtain from it advantages of any kind.
3. Similarly, to reaffirm the obligation of these states to refrain from organizing, supporting, promoting, financing, instigating, or tolerating subversive, terrorist, or armed activities against another state and from intervening in a civil war in another state or in its internal struggles."
The general rule prohibiting force allows for certain exceptions. In view of the arguments advanced by the United States to justify the acts of which it is accused by Nicaragua, the Court must express a view on the content of the right of self-defence, and more particularly the right of collective self-defence. First, with regard to the existence of this right, it notes that in the language of Article 51 of the United Nations Charter, the inherent right (or "droit naturel") which any State possesses in the event of an armed attack, covers both collective and individual self-defence. Thus, the Charter itself testifies to the existence of the right of collective self-defence in customary international law. Moreover, just as the wording of certain General Assembly declarations adopted by States demonstrates their recognition of the principle of the prohibition of force as definitely a matter of customary international law, some of the wording in those declarations operates similarly in respect of the right of self-defence (both collective and individual). Thus, in the declaration quoted above on the Principles of International Law concerning Friendly Relations and Co-operation among States in accordance with the Charter of the United Nations, the reference to the prohibition of force is followed by a paragraph stating that:

"nothing in the foregoing paragraphs shall be construed as enlarging or diminishing in any way the scope of the provisions of the Charter concerning cases in which the use of force is lawful".

This resolution demonstrates that the States represented in the General Assembly regard the exception to the prohibition of force constituted by the right of individual or collective self-defence as already a matter of customary international law.

[p. 108] 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.

[pp. 118-119] As to the claim that United States activities in relation to the contras constitute a breach of the customary international law principle of the non-use of force, the Court finds that, subject to the question whether the action of the United States might be justified as an exercise of the right of self-defence, the United States has committed a prima facie violation of that principle by its assistance to the contras in Nicaragua, by "organizing or encouraging the organization of irregular forces or armed bands ... for incursion into the territory of another State", and "participating in acts of civil strife ... in another State", in the terms of General Assembly resolution 2625 (XXV). According to that resolution, participation of this kind is contrary to the principle of the prohibition of the use of force when the acts of civil strife referred to "involve a threat or use of force". In the view of the Court, while the arming and training of the contras can certainly be said to involve the threat or use of force against Nicaragua, this is not necessarily so in respect of all the assistance given by the United States Government. In particular, the Court considers that the mere supply of funds to the contras while undoubtedly an act of intervention in the internal affairs of Nicaragua, as will be explained below, does not in itself amount to a use of force.

[p. 127] On the legal level the Court cannot regard response to an intervention by Nicaragua as such a justification. While an armed attack would give rise to an entitlement to collective self-defence, a use of force of a lesser degree of gravity cannot, as the Court has already observed (paragraph 211 above), produce any entitlement to take collective counter-measures involving the use of force. The acts of which Nicaragua is accused, even assuming them to have been established and imputable to that State, could only have justified proportionate counter-measures on the part of the State which had been the victim of these acts, namely El Salvador, Honduras or Costa Rica. They could not justify counter-measures taken by a third State, the United States, and particularly could not justify intervention involving the use of force.

[p. 128] The effects of the principle of respect for territorial sovereignty inevitably overlap with those of the principles of the prohibition of the use of force and of non-intervention. Thus the assistance to the contras, as well as the direct attacks on Nicaraguan ports, oil installations, etc., referred to in paragraphs 81 to 86 above, not only amount to an unlawful use of force, but also constitute infringements of the territorial sovereignty of Nicaragua, and incursions into its territorial and internal waters. Similarly, the mining operations in the Nicaraguan ports not only constitute breaches of the principle of the non-use of force, but also affect Nicaragua's sovereignty over certain maritime expanses. The Court has in fact found that these operations were carried on in Nicaragua's territorial or internal waters or both (paragraph 80), and accordingly they constitute a violation of Nicaragua's sovereignty. The principle of respect for territorial sovereignty is also directly infringed by the unauthorized overflight of a State's territory by aircraft belonging to or under the control of the government of another State. The Court has found above that such overflights were in fact made (paragraph 91 above).
These violations cannot be justified either by collective self-defence, for which, as the Court has recognized, the necessary circumstances are lacking, nor by any right of the United States to take counter-measures involving the use of force in the event of intervention by Nicaragua in El Salvador, since no such right exists under the applicable international law. They cannot be justified by the activities in El Salvador attributed to the Government of Nicaragua. The latter activities, assuming that they did in fact occur, do not bring into effect any right belonging to the United States which would justify the actions in question. Accordingly, such actions constitute violations of Nicaragua's sovereignty under customary international law.

[pp. 151-152 S.O. Singh] A major consideration in the resolution of the dispute in this case has been the principle of non-use of force. It is indeed a well-established tenet of modern international law that the lawful use of force is circumscribed by proper regulation, and this is so from whichever angle one looks at it, whether the customary viewpoint or that of the conventional international law on the subject. However the customary aspect does visualize the exceptional need for the provision of the "inherent right" to use force in self-defence. The aforesaid concepts of the principle and its exception do have an existence independent of treaty-law as contained in the United Nations Charter or the Inter-American system of conventional law on the subject. In this context it appears necessary to emphasize certain aspects, which is attempted below.

In fact this cardinal principle of non-use of force in international relations has been the pivotal point of a time-honoured legal philosophy that has evolved particularly after the two World Wars of the current century. It has thus been deliberately extended to cover the illegality of recourse to armed reprisals or other forms of armed intervention not amounting to war which aspect may not have been established by the law of the League of Nations, or by the Nuremberg or Tokyo Trials, but left to be expressly developed and codified by the United Nations Charter. The logic behind this extension of the principle of non-use of force to reprisals has been that if use of force was made permissible not as a lone restricted measure of self-defence, but also for other minor provocations demanding counter-measures, the day would soon dawn when the world would have to face the major catastrophe of a third World War - an event so dreaded in 1946 as to have justified concrete measures being taken forthwith to eliminate such a contingency arising in the future.

There can be no doubt therefore of the innate legal existence of this basic reasoning, irrespective of the later developments which have now found a place in the treaty provisions as reflected in Article 2, paragraph 4, and Article 51 of the United Nations Charter. However it is pertinent that the origin of legal regulation of use of force is much older than the United Nations Charter and this has been acknowledged to be so. If an issue was raised whether the concepts of the principle of non-use of force and the exception to it in the form of use of force for self-defence are to be characterized as either part of customary international law or that of conventional law, the answer would appear to be that both the concepts are inherently based in customary international law in their origins, but have been developed further by treaty-law. In any search to determine whether these concepts belong to customary or conventional international law it would appear to be a fallacy to try to split any concept to ascertain what part or percentage of it belongs to customary law and what fraction belongs to conventional law. There is no need to try to separate the inseparable, because the simple logical approach would be that if the concept in its origin was a customary one, as in this case, and later built up by treaty law, the Court would be right in ruling that the present dispute before the Court does not arise under a multilateral treaty, so as to fall outside the Court's jurisdiction because of the Vandenberg Reservation invoked by the Respondent.

[p. 176 S.O. Ruda] If, juridically, assistance to rebels cannot, per se, be justified on grounds of self-defence, I do not see why the Court feels bound to analyse in detail the facts of the case relating to such assistance. Neither do I perceive the need for entering, in the Judgment, into the questions of the requirements, in the case of collective self-defence, of a request by a State which regards itself as the victim of an armed attack, or a declaration by that State that it has been attacked or of its submission of an immediate report on the measure taken in the exercise of this right of self-defence.
From my point of view it would have been sufficient to say, just as the Court does in its conclusions, that even if there was such assistance and flow of arms, that is not a sufficient excuse for invoking self-defence because, juridically, the concept of "armed attack" does not include assistance to rebels.

[pp. 530-531 D.O. Jennings] Let us look first, therefore, at the relationship between customary international law, and Article 2, paragraph 4, and Article 51 of the United Nations Charter. There is no doubt that there was, prior to the United Nations Charter, a customary law which restricted the lawful use of force, and which correspondingly provided also for a right to use force in self-defence; as indeed the use of the term "inherent" in Article 51 of the United Nations Charter suggests. The proposition, however, that, after the Charter, there exists alongside those Charter provisions on force and self-defence, an independent customary law that can be applied as alternative to Articles 2, paragraph 4, and 51 of the Charter, raises questions about how and when this correspondence came about, and about what the differences, if any, between customary law and the Charter provisions, may be.

A multilateral treaty may certainly be declaratory of customary international law either:

"as incorporating and giving recognition to a rule of customary international law that existed prior to the conclusion of the treaty or, on the other hand, as being the fons et origo of a rule of international law which subsequently secured the general assent of States and thereby was transformed into customary law" (see Baxter, British Year Book of International Law, Vol. XLI, 1965-1966, p. 277).

It could hardly be contended that these provisions of the Charter were merely a codification of the existing customary law. The literature is replete with statements that Article 2, paragraph 4, - for example in speaking of "force" rather than war, and providing that even a "threat of force" may be unlawful - represented an important innovation in the law. The late Sir Humphrey Waldock, in a passage dealing with matters very much in issue in the present case, put it this way:

"The illegality of recourse to armed reprisals or other forms of armed intervention not amounting to war was not established beyond all doubt by the law of the League, or by the Nuremberg and Tokyo Trials. That was brought about by the law of the Charter ..." (106 Collected Courses, Academy of International Law, The Hague (1962-II), p. 231.)

Even Article 51, though referring to an "inherent" and therefore supposedly pre-existing, right of self-defence, introduced a novel concept in speaking of "collective self-defence" 1. Article 51 was introduced into the Charter at a late stage for the specific purpose of clarifying the position in regard to collective understandings - multilateral treaties - for mutual self-defence, which were part of the contemporary scene.

1 Cf. Aréchaga, 159 Collected Courses, The Hague (1978-I), at p. 87, and p. 96 where he goes so far as to assert: "The so-called customary law of self-defence supposedly pre-existing the Charter, and dependent on this single word [inherent] simply did not exist."