In 1917, Lenin and many other revolutionaries returned from exile when they were given permission by the Germans to cross Germany. The Germans hoped they would undermine support for the government and end Russian involvement in World War I. Lenin pushed the then small Bolshevik party into action, with campaigns aimed at ending the war. The failure of Russia's all-out offensive in the war further undermined support for the provisional government, leading many to encourage the soviet to seize power. The Bolsheviks took control of this movement, and despite being suppressed by the government, succeeded in gaining popular support.

The Bolsheviks aided the provisional government in defeating a coup attempt by General Kornilov, leading them to eventually gain a majority in the Petrograd Soviet. On the night of Nov. 6th (Oct. 24th in the Russian calendar), the Bolsheviks staged a coup d'etat, engineered by Leon Trotsky, and affirmed the seizure of power by a vote in the soviet (after the Mensheviks and Socialist Revolutionaries walked out). They then formed a cabinet, known as the Council of People's Commissars, with Lenin as chairman, Trotsky as foreign commissar, Aleksey Rykov as interior commissar, and Josef Stalin as commissar of nationalities. They then negotiated an end to WWI, with the humiliating Treaty of Brest-Litovsk.

See also: Russian Civil War