Suspension of the Intermediate-Range Nuclear Forces Treaty prior to its withdrawal: the content of custom

The United States suspended the Intermediate-Range Nuclear Forces Treaty (INF Treaty) ‘in accordance with customary international law’. However, State practice prior to the International Law Commission's codification of the law of treaties did not contribute to clarifying the extent of a right to suspend and the proper conditions for its exercise under customary international law. The few instances regarding suspension due to a serious breach did not demonstrate how the treaties in question were suspended but were a mere reference to a right of suspension in diplomatic or political documents. Against that backdrop, this article seeks to delineate what customary rules the United States believed it was observing and to clarify to what extent those rules are identical to or different from the codified rules on suspension in the Vienna Convention on the Law of Treaties (Convention). Because the codified procedural safeguards or the mechanism of acquiescence under Article 65 of the Convention were considered as the progressive development of international law, it appears possible to suspend the INF Treaty unilaterally outside the Convention and under the customary rules by which the United States is bound. The INF Treaty was suspended by the United States and by Russia in sequence. That Russian suspension appears to have been an exceptio non adimpleti contractus to prevent the asymmetric execution of the INF Treaty that had been previously suspended by the United States.