The combined Soviet civilian and military casualty count - estimated to be around 20 million people - accounted for the majority of losses of Allied forces. After the outbreak of World War II, Germany invaded the Soviet Union. Stalin conducted the Great Purge to remove his actual and perceived opponents. The forced labour camp system of the Gulag was also expanded in this period.

Stalin inaugurated a period of rapid industrialization and forced collectivization that led to significant economic growth, but also contributed to a famine in 1930–1933 that killed millions. Following Lenin's death in 1924, Joseph Stalin came to power.

As the war progressed in the Bolsheviks' favor, the RSFSR began to incorporate land conquered from the war into nominally independent states, which were unified into the Soviet Union in December 1922. Persisting internal tensions escalated into the brutal Russian Civil War. The new government, led by Vladimir Lenin, established the Russian Soviet Republic, the world's first constitutionally guaranteed socialist state. The country's roots lay in the October Revolution of 1917, which saw the Bolsheviks overthrow the Russian Provisional Government that formed earlier that year following the February Revolution and the abdication of Tsar Nicholas II, marking the end of the Russian Empire. As a one-party state governed by the Communist Party of the Soviet Union, it was a flagship communist state. The country was a successor state to the Russian Empire it was nominally a federal union of fifteen national republics, the largest and most populous of which was the Russian SFSR, but in practice both its government and its economy were highly centralized until its final years. The Soviet Union, officially the Union of Soviet Socialist Republics ( USSR), was a transcontinental country that spanned much of Eurasia from 1922 to 1991.

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