The Soviet Union, officially the Union of Soviet Socialist Republics (USSR) and also known unofficially as Russia, was a socialist state in Eurasia that existed from 1922 to 1991. Nominally a political union of multiple equal national Soviet republics and thus comparable to today's concept of a supranational union, the state had in reality many strong unitary aspects. Although each republic had its own communist party, the union was a one-party state governed by the All-Union Communist Party. Moscow was the capital of both the union and its largest republic, the Russian Soviet Federative Socialist Republic. The Russian nation had constitutionally equal status among the many nations of the union but exerted de facto dominance in various respects.
The Soviet Union had its roots in the October Revolution of 1917, when the Bolsheviks, led by Vladimir Lenin, overthrew the Russian Provisional Government which had replaced Tsar Nicholas II. In 1922, the Soviet Union was formed with the unification of the Russian, Transcaucasian, Ukrainian, and Byelorussian republics. Following Lenin's death in 1924 and a brief power struggle, Joseph Stalin came to power in the mid-1920s. Stalin committed the state's ideology to Marxism–Leninism (which he created), and initiated a centrally planned economy. As a result, the country underwent a period of rapid industrialization and collectivization. Political paranoia was also fomented around Stalin, and the Great Purge was carried out to remove his opponents from the Communist Party through arbitrary arrests, prosecutions & executions on hundreds of thousands of people.