The Union of Soviet Socialist Republics (Russian: Сою́з Сове́тских Социалисти́ческих Респу́блик, tr. Soyuz Sovetskikh Sotsialisticheskikh Respublik) abbreviated to USSR (Russian: СССР, tr. SSSR) or the Soviet Union (Russian: Советский Союз, tr. Sovetsky Soyuz), was a constitutionally communist state that existed between 1922 and 1991, ruled as a single-party state by the Communist Party with Moscow as its capital. A union of multiple subnational Soviet republics, its government and economy were highly centralized.

The Soviet Union had its roots in the Russian Revolution of 1917, which deposed the imperial autocracy. The Bolsheviks, led by Vladimir Lenin, then overthrew the Provisional Government. The Russian Socialist Federative Soviet Republic was established and the Russian Civil War began. The Red Army entered several territories of the former Russian Empire and helped local communists seize power. In 1922, the Bolsheviks were victorious, forming the Soviet Union with the unification of the Russian, Transcaucasian, Ukrainian and Byelorussian republics. Following Lenin's death in 1924, a troika collective leadership and a brief power struggle, Joseph Stalin came to power in the mid-1920s. Stalin committed the state ideology to Marxism–Leninism and initiated a centrally planned economy. As a result, the country underwent a period of rapid industrialisation and collectivisation which laid the basis for its later war effort and dominance after World War II. However, Stalin repressed both Communist Party members and elements of the population through his authoritarian rule.