Treaty on the Creation of the Union of Soviet Socialist Republics

The Declaration and Treaty on the Formation of the Union of Soviet Socialist Republics (Russian: Декларация и договор об образовании Союза Советских Социалистических Республик) officially created the Union of Soviet Socialist Republics (USSR), commonly known as the Soviet Union. It de jure legalised a political union of several Soviet republics that had existed since 1919 and created a new federal government whose key functions were centralised in Moscow.

Source: Wikipedia — Treaty on the Creation of the Union of Soviet Socialist Republics (CC BY-SA 4.0)