The Union of Soviet Socialist Republics (USSR), and commonly known as the Soviet Union was a constitutionally socialist state that existed in Eurasia between 1922 and 1991. A more informal name also used among its residents was the Union (Soyuz).
The Soviet Union had a single-party political system dominated by the Communist Party until 1990. 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.