The Soviet Union, officially the Union of Soviet Socialist Republics (USSR), was a socialist state that spanned Europe and Asia during its existence from 1922 to 1991. It was nominally a federal union of multiple national republics; in practice its government and economy were highly centralized until its final years. The country was a one-party state prior to 1990 governed by the Communist Party of the Soviet Union, with Moscow as its capital within its largest and most populous republic, the Russian SFSR. Other major urban centers were Leningrad (Russian SFSR), Kiev (Ukrainian SSR), Minsk (Byelorussian SSR), Tashkent (Uzbek SSR), Alma-Ata (Kazakh SSR) and Novosibirsk (Russian SFSR). It was the largest country in the world, covering over 22,402,200 square kilometres (8,649,500 sq mi), and spanning eleven time zones.
The Soviet Union had its roots in the October Revolution of 1917 when the Bolsheviks, headed by Vladimir Lenin, overthrew the Provisional Government that had earlier replaced the monarchy of the Russian Empire. They established the Russian Soviet Republic, beginning a civil war between the Bolshevik Red Army and many anti-Bolshevik forces across the former Empire, among whom the largest faction was the White Guard, which engaged in violent anti-communist repression against the Bolsheviks and their worker and peasant supporters known as the White Terror. The Red Army expanded and helped local Bolsheviks take power, establishing soviets, repressing their political opponents and rebellious peasants through Red Terror. By 1922, the Bolsheviks had emerged victorious, forming the Soviet Union with the unification of the Russian, Transcaucasian, Ukrainian and Byelorussian republics. The New Economic Policy (NEP), which was introduced by Lenin, led to a partial return of a free market and private property; this resulted in a period of economic recovery.