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World Court Digest

I. Substantive International Law - First Part
2.1. General Questions
2.1.5. Relation between the Sources of International Law

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

[pp. 93-94] The Court would observe that, according to the United States argument, it should refrain from applying the rules of customary international law because they have been "subsumed" and "supervened" by those of international treaty law, and especially those of the United Nations Charter. Thus the United States apparently takes the view that 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.
The Court does not consider that, in the areas of law relevant to the present dispute, it can be claimed that all the customary rules which may be invoked have a content exactly identical to that of the rules contained in the treaties which cannot be applied by virtue of the United States reservation. On a number of points, the areas governed by the two sources of law do not exactly overlap, and the substantive rules in which they are framed are not identical in content. But in addition, even if a treaty norm and a customary norm relevant to the present dispute were to have exactly the same content, this would not be a reason for the Court to take the view that the operation of the treaty process must necessarily deprive the customary norm of its separate applicability. Nor can the multilateral treaty reservation be interpreted as meaning that, once applicable to a given dispute, it would exclude the application of any rule of customary international law the content of which was the same as, or analogous to, that of the treaty-law rule which had caused the reservation to become effective.

[pp. 94-96] As regards the suggestion that the areas covered by the two sources of law are identical, the Court observes that the United Nations Charter, the convention to which most of the United States argument is directed, by no means covers the whole area of the regulation of the use of force in international relations. On one essential point, this treaty itself refers to pre-existing customary international law; this reference to customary law is contained in the actual text of Article 5l, which mentions the "inherent right" (in the French text the "droit naturel") of individual or collective self-defence, which "nothing in the present Charter shall impair" and which applies in the event of an armed attack. The Court therefore finds that Article 51 of the Charter is only meaningful on the basis that there is a "natural" or "inherent" right of self-defence, and it is hard to see how this can be other than of a customary nature, even if its present content has been confirmed and influenced by the Charter. Moreover the Charter, having itself recognized the existence of this right, 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, authorizes 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. It rather demonstrates that in the field in question, the importance of which for the present dispute need hardly be stressed, customary international law continues to exist alongside treaty law. The areas governed by the two sources of law thus do not overlap exactly, and the rules do not have the same content. This could also be demonstrated for other subjects, in particular for the principle of non-intervention.
But as observed above (paragraph 175), even if the customary norm and the treaty norm were to have exactly the same content, this would not be a reason for the Court to hold that the incorporation of the customary norm into treaty-law must deprive the customary norm of its applicability as distinct from that of the treaty norm. The existence of identical rules in international treaty law and customary law has been clearly recognized by the Court in the North Sea Continental Shelf cases. To a large extent, those cases turned on the question whether a rule enshrined in a treaty also existed as a customary rule, either because the treaty had merely codified the custom, or caused it to "crystallize", or because it had influenced its subsequent adoption. The Court found that this identity of content in treaty law and in customary international law did not exist in the case of the rule invoked, which appeared in one article of the treaty, but did not suggest that such identity was debarred as a matter of principle: on the contrary, it considered it to be clear that certain other articles of the treaty in question "were ... regarded as reflecting, or as crystallizing, received or at least emergent rules of customary international law" (I.C.J. Reports 1969, p. 39. para. 63). More generally, there are no grounds for holding that when customary international law is comprised of rules identical to those of treaty law, the latter "supervenes" the former, so that the customary international law has no further existence of its own.
There are a number of reasons for considering that, even if two norms belonging to two sources of international law appear identical in content, and even if the States in question are bound by these rules both on the level of treaty-law and on that of customary international law, these norms retain a separate existence. This is so from the standpoint of their applicability. In a legal dispute affecting two States, one of them may argue that the applicability of a treaty rule to its own conduct depends on the other State's conduct in respect of the application of other rules, on other subjects, also included in the same treaty. For example, if a State exercises its right to terminate or suspend the operation of a treaty on the ground of the violation by the other party of a "provision essential to the accomplishment of the object or purpose of the treaty" (in the words of Art. 60, para. 3 (b), of the Vienna Convention on the Law of Treaties), it is exempted, vis-à-vis the other State, from a rule of treaty-law because of the breach by that other State of a different rule of treaty-law. But if the two rules in question also exist as rules of customary international law, the failure of the one State to apply the one rule does not justify the other State in declining to apply the other rule. Rules which are identical in treaty law and in customary international law are also distinguishable by reference to the methods of interpretation and application. A State may accept a rule contained in a treaty not simply because it favours the application of the rule itself, but also because the treaty establishes what that State regards as desirable institutions or mechanisms to ensure implementation of the rule. Thus, if that rule parallels a rule of customary international law, two rules of the same content are subject to separate treatment as regards the organs competent to verify their implementation, depending on whether they are customary rules or treaty rules. The present dispute illustrates this point.
It will therefore be clear that customary international law continues to exist and to apply, separately from international treaty law, even where the two categories of law have an identical content. Consequently, in ascertaining the content of the customary international law applicable to the present dispute, the Court must satisfy itself that the Parties are bound by the customary rules in question; but the Court is in no way bound to uphold these rules only in so far as they differ from the treaty rules which it is prevented by the United States reservation from applying in the present dispute.

[pp. 96-97] The question raised by this argument is whether the provisions of the multilateral treaties in question, particularly the United Nations Charter, diverge from the relevant rules of customary international law to such an extent that a judgment of the Court as to the rights and obligations of the parties under customary law, disregarding the content of the multilateral treaties binding on the parties, would be a wholly academic exercise, and not "susceptible of any compliance or execution whatever" (Northern Cameroons, I.C.J. Reports 1963, p. 37). The Court does not consider that this is the case. As already noted, on the question of the use of force, the United States itself argues for a complete identity of the relevant rules of customary international law with the provisions of the Charter. The Court has not accepted this extreme contention, having found that on a number of points the areas governed by the two sources of law do not exactly overlap, and the substantive rules in which they are framed are not identical in content (paragraph 174 above). 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, or a judgment not susceptible of compliance or execution.

[p. 207 S.O. Ni] However, it is certain that when principles of customary international law are incorporated into a multilateral treaty like the United Nations Charter, these principles of customary international law do not thereby become extinct. The same principles continue to be operative and binding on States, sometimes alongside or in conjunction with treaty law, in their international relations with one another. Article 38, paragraph 1, of the Statute enumerates, as applicable by the Court, the various sources of international law which, in the course of application, usually support, rather than preclude, each other. But it would be inconceivable that application of one should exclude that of any other.

[p. 303 D.O. Schwebel] The argument that the principles if not the provisions of the United Nations Charter governing the use of force in international relations have been incorporated into the body of customary international law is widely and authoritatively accepted, despite the fact that the practice of States manifests such irregular support for the principles of law which the Charter proclaims. Indeed, it could even be argued that the practice, in contrast to the preachment, of States indicates that the restrictions on the use of force in international relations found in the Charter are not part of customary international law.

[p. 305 D.O. Schwebel] ... the Court is bound to give substantive effect to the multilateral treaty reservation. It is not free to avoid its application by an argument which, if technically defensible, in real terms would vitiate a limitation which the United States has imposed upon the jurisdiction of the Court. Accordingly, while recognizing that there is room for the contrary conclusion which the Court has reached, I conclude that the generally accepted essential, even if incomplete, identity of Charter principles and principles of customary international law on the use of force in international relations, rather than authorizing the Court to apply those customary principles to the central issues of this case, precludes the Court from doing so by reason of the limitations imposed upon the Court's jurisdiction by the multilateral treaty reservation.

[p. 531 D.O. Jennings] If, then, the Charter was not a codification of existing custom about force and self-defence, the question must then be asked whether a general customary law, replicating the Charter provisions, has developed as a result of the influence of the Charter provisions, coupled presumably with subsequent and consonant States' practice; so that it might be said that these Charter provisions:

"generated a rule which, while only conventional or contractual in its origin, has since passed into the general corpus of international law, and is now accepted as such by the opinio juris, so as to have become binding even for countries which have never, and do not, become parties to the Convention" (I.C.J. Reports 1969, p. 41, para. 71).

But there are obvious difficulties about extracting even a scintilla of relevant "practice" on these matters from the behaviour of those few States which are not parties to the Charter; and the behaviour of all the rest, and the opinio juris which it might otherwise evidence, is surely explained by their being bound by the Charter itself 1.
There is, however, a further problem: the widely recognized special status of the Charter itself. This is evident from paragraph 6 of Article 2, that:

"The Organization shall ensure that States which are not Members of the United Nations act in accordance with these Principles so far as may be necessary for the maintenance of international peace and security."

This contemplates obligations for non-members arising immediately upon the coming into operation of the Charter, which obligations could at that time only be derived, like those for Members, directly from the Charter itself. Even "instant" custom, if there be such a thing, can hardly be simultaneous with the instrument from which it develops. There is, therefore, no room and no need for the very artificial postulate of a customary law paralleling these Charter provisions.

[pp. 532-533 D.O. Jennings] This use of treaty provisions as "evidence" of custom, takes the form of an interpretation of the treaty text. Yet the Court itself acknowledges that treaty-law and customary law can be distinguished precisely because the canons of interpretation are different (para. 178). To indulge the treaty interpretation process, in order to determine the content of a posited customary rule, must raise a suspicion that it is in reality the treaty itself that is being applied under another name. Of course this way of going about things may be justified where the treaty text was, from the beginning, designed to be a codification of custom; or where the treaty is itself the origin of a customary law rule. But, as we have already seen, this could certainly not be said of Article 2, paragraph 4, or even Article 51, of the United Nations Charter; nor indeed of most of the other relevant multilateral treaty provisions.
The reader cannot but put to himself the question whether the Judgment would, in its main substance, have been noticeably different in its content and argument, had the application of the multilateral treaty reservation been rejected.
There is no need to pursue further the relationship of the United Nations Charter and customary law; for even if a different view of this question could be adopted, there remains, quite independently, a most cogent objection to any attempt to decide the issues of force and self-defence without the Charter of the United Nations or other relevant treaties. Although the multilateral treaty reservation qualifies the jurisdiction of this Court, it does not qualify the substantive law governing the behaviour of the Parties at the material times. Article 38 of the Court's own Statute requires it first to apply "international conventions, general" as well as "particular" ones, "establishing rules expressly recognized by the contesting States"; and the relevant provisions of the Charter - and indeed also of the Charter of the Organization of American States, and of the Rio Treaty - have at all material times been principal elements of the applicable law governing the conduct, rights and obligations of the Parties. It seems, therefore, eccentric, if not perverse, to attempt to determine the central issues of the present case, after having first abstracted these principal elements of the law applicable to the case, and which still obligate both the Parties.

1For an assessment of this important question, especially in relation to the Declaration of Principles of Friendly Relations, see Professor Arangio-Ruiz, 137 Collected Courses, The Hague (1972-III), Chap. IV.