Officially the Union of Soviet Socialist Republics. Союз Советских Социалистических Республик

The Soviet Union had its roots in the October Revolution of 1917, when the Bolsheviks, led by Vladimir Lenin, overthrew the Russian Provisional Government that had replaced Tsar Nicholas II. They established the Russian Socialist Federative Soviet Republic (renamed Russian Soviet Federative Socialist Republic in 1936), beginning a civil war between the revolutionary "Reds" and the counter-revolutionary "Whites." The Red Army entered several territories of the former Russian Empire and helped local communists take power through soviets, which nominally acted on behalf of workers and peasants. In 1922, the communists 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 and a brief power struggle, Joseph Stalin came to power in the mid-1920s. Stalin suppressed all political opposition to his rule, committed the state ideology to Marxism–Leninism (which he created), and initiated a centrally planned command economy. As a result, the country underwent a period of rapid industrialization and collectivization which laid the foundation for its victory in World War II and post-war dominance of Eastern Europe. Stalin also fomented political paranoia, and conducted the Great Purge to remove opponents of his from the Communist Party through the mass arbitrary arrest of many people (military leaders, Communist Party members, and ordinary citizens alike) who were then sent to correctional labor camps or sentenced to death.