The Soviet Union had a single-party political system dominated by the Communist Party. Although the USSR was nominally a union of Soviet republics (15 in all after 1956) with the capital in Moscow, it was actually a highly centralized state with a planned economy.
The Russian Revolution of 1917 brought about the downfall of the Russian Empire. Its successor, the Russian Provisional Government, was short-lived. After the Bolsheviks won the ensuing Russian Civil War, the Soviet Union was founded in December 1922 with the merger of the Russian Soviet Federative Socialist Republic, Transcaucasian Socialist Federative Soviet Republic, Ukrainian Soviet Socialist Republic and Byelorussian Soviet Socialist Republic.
Following the death of the first Soviet leader, Vladimir Lenin, in 1924, Joseph Stalin eventually won a power struggle and led the country through a large-scale industrialization with a command economy and political repression. In World War II, in June 1941, Germany and its allies invaded the Soviet Union, a country with which it had signed a non-aggression pact. After four years of brutal warfare, the Soviet Union emerged victorious as one of the world's two superpowers, the other being the United States.