On 23 May 1969, in Vienna, the Vienna Convention on the Law of Treaties is adopted — the modern codification of rules on concluding, interpreting, applying and terminating treaties between states.

The UN conference (1968–1969) built on the International Law Commission draft. The text covers consent, reservations, pacta sunt servanda, interpretation (Articles 31–33) and invalidity for error, fraud, coercion or conflict with ius cogens.

Entry into force: 27 January 1980. Even non-parties invoke many provisions as customary law; the International Court of Justice cites them frequently.

The Convention does not cover treaties of international organisations (addressed separately in 1986) or state succession. The depositary is the UN Secretary-General.

For diplomacy and boundary disputes, Vienna 1969 became the reference manual of the “law of treaties”.

The official text appears in the UNTC, Chapter XXIII, and in the United Nations Treaty Series.

The Vienna Convention of 23 May 1969 remains the cornerstone of the law of treaties between states.

Image: Vienna; Wikimedia Commons. Cropped to 16:9.

Source consulted: Vienna Convention on the Law of Treaties. Treaty text: Vienna Convention on the Law of Treaties (UNTC).