The Vienna Convention on the Law of Treaties, adopted on May 22 1969, codified the pre-existing international customary law on treaties, with some necessary gap-filling and clarifications. Most states are parties to it. However, even those that are not may still recognize it as binding upon them in as much as it is a restatement of already existing law (the United States is among these).
The Convention entered into force on 27 January 1980.
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