As regards the United States in particular, the weight of an expression of opinion 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).
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