Treaties are governed by international law and are a primary source of international law. Treaties play an important role in the orderly conduct of relations among states, particularly following World War II. In order for treaties to perform this role, internationally recognized rules concerning treaties have been developed.

The Vienna Convention on the Law of Treaties, concluded at Vienna on May 23, 1969 (VCLT), sets forth rules concerning the making, operation, suspension, and termination of treaties. The VCLT opened for signature in Vienna in May 1969, and it entered into force on January 27, 1980. Currently, over 90 countries have signed or acceded to the VCLT.

The VCLT was signed for the United States on April 24, 1970, and President Nixon transmitted the treaty to the Senate for its advice and consent to ratification on November 22, 1971. The VCLT remains before the Senate Foreign Relations Committee. The United States has not ratified the VCLT and thus is not legally bound by its provisions. Nevertheless, the United States follows many of the rules in the VCLT in the conduct of its day-to-day work on treaties.

States enter into treaties not only with other states, but also with other subjects of international law (in particular, international organizations). International organizations enter into treaties with each other. These treaties are not covered by the VCLT, but are the subject of the Vienna Convention on the Law of Treaties between States and International Organizations or between International Organizations, 1986 (VCLTSIO). This treaty is not yet in force, although many of its provisions are based on those in the VCLT and have been adapted for international organizations.

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