After the October Revolution the new Russian Soviet Federative Socialist Republic reconquered most of the former Tsarist territory during the Civil War of 1918-1922. By the end of that war Moscow stood at the centre and the Ukrainian, Belarusian and Transcaucasian Soviet Republics on the periphery. They were formally independent states but were already integrated through a shared army, a shared currency and a shared Communist Party. The remaining question was constitutional form.

Here Lenin and Stalin sharply diverged. Stalin's 'autonomisation' plan would have folded the other republics into the RSFSR as autonomous units — Russia would have expanded. Lenin, from his sickbed, opposed this and pushed through an 'equal federation' model. On 30 December 1922 the First Congress of Soviets ratified the Treaty of Union; the USSR came into being. The 1924 Constitution wrote federalism and a 'right of secession' onto paper. In practice a single party — the All-Union Communist Party — drained that paper of meaning: real power sat with the Politburo in Moscow.

After Lenin died in 1924, Stalin sidelined Trotsky and the other contenders and concentrated power in himself. From 1928 the Five-Year Plans began: heavy industry, steel, tractors and dam projects pushed forward at era-changing speed. Agriculture was forcibly collectivised; the 1932-33 famine in Ukraine and Kazakhstan killed millions. The Great Terror of 1936-38 purged the party, army and intelligentsia. The same USSR was the power that resisted the Nazi invasion in 1941 and entered Berlin in 1945 — the backbone of the anti-fascist coalition.

After the war the Soviet Union sat at the centre of a bloc that included Eastern Europe, the Chinese Revolution, Vietnam and Cuba; the Cold War ran along this axis for forty years. The Soviet planned economy delivered dramatic industrialisation but never caught up with the West in consumer goods, agricultural productivity or information technology. Gorbachev's perestroika and glasnost reforms from 1985 coincided with the 1989 collapse of the Eastern Bloc; by the end of 1991 the USSR itself broke into fifteen independent republics. It lasted seventy years, shaped the 20th century, and its legacy — the tensions of federalism, the nostalgic or critical interest in the planned economy, the politics of the post-Soviet space — remains open today.