When Tsar Nicholas II was overthrown in February 1917 Russia was in deep crisis: grinding defeats in the First World War, famine, waves of urban strikes, armies disintegrating at the front. The Provisional Government that replaced him was a liberal-socialist coalition, but workers' and soldiers' councils (soviets) had organised in parallel. This 'dual power' lasted months. In April, returning from Swiss exile, Lenin laid out a sharp programme in his 'April Theses': end the imperialist war, land to the peasants, factories to the workers, 'all power to the soviets'.

In March the Bolsheviks were a small minority. As the Provisional Government lost legitimacy by trying to continue the war, their weight in the councils grew; by September they held a majority in the Petrograd Soviet. On the night of 24-25 October (Julian calendar) the Military Revolutionary Committee, run by Trotsky, seized the bridges, telegraphs and stations of Petrograd; they entered the Winter Palace and arrested Kerensky's cabinet. The next day the Second All-Russian Congress of Soviets transferred all power to the soviets and proclaimed the decrees on peace and land.

The years that followed were a bloody transformation. In 1918 one-party rule was consolidated, the Constituent Assembly dissolved, the Treaty of Brest-Litovsk signed with Germany, the imperial family executed. In the Civil War of 1918-1922 the Red Army pushed back the Whites and the fourteen foreign powers backing them. The cost was millions of lives: war deaths, famine, the Cheka's 'Red Terror'. The USSR was founded in 1922; when Lenin died in 1924 the winner of the succession struggle was Stalin.

The October Revolution's historical readings remain contested. One tradition reads it as an emancipatory rupture against exploitation; another as a small minority's coup that seeded the totalitarianism to come. Whether Stalin's terror grew out of, or broke with, the Lenin-Trotsky years is itself debated. What is unambiguous is that October 1917 placed an alternative model of social organisation onto the field of modernity, and from anti-colonial movements to European social democracy, from the Cold War to the Chinese Revolution, became a defining axis of the 20th century.