As regards the United States in particular, the weight oof 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 State 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.
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