On the 30th December 1922, the USSR – the Union of Soviet Socialist Republics – was founded. The Treaty on the Creation of the USSR and the Declaration of the Creation of the USSR, which were approved by delegations from the founding countries on the 29th December, formed the constitutional basis for the Soviet Union. However, they didn’t officially come into force until the 30th when they were confirmed by the 1st Congress of Soviets and signed by the heads of each republic’s respective delegation.
The Soviet Union in 1922 consisted of just four Soviet republics – the Russian SFSR, Ukrainian SSR, Byelorussian SSR and Transcaucasian SFSR – although it’s important to note that the Russian and Transcaucasian SFSRs actually incorporated a number of separate Soviet Socialist Republics. The creation of the USSR therefore effectively created a centralised federal government.
This was an important step for the Bolsheviks who, having won the Russian Civil War, needed to consolidate their gains into a formal political entity. Stalin in particular argued that the New Economic Policy that followed war communism required centralised control, which threatened some national groups. At the same time, some Bolsheviks hoped for a world revolution that would overthrow capitalist governments around the globe.
The USSR’s founding documents therefore allowed Soviet republics to withdraw from the Union at any time, even though none of them actually did so before the collapse of the Soviet Union in 1991. Similarly new members were able to join the union at any time, which meant that by 1940 the USSR’s membership had grown from four republics in 1922 to 16.