Stalin was one of the seven members of the first Politburo, founded in 1917 in order to manage the Bolshevik Revolution, alongside Lenin, Zinoviev, Kamenev, Trotsky, Sokolnikov and Bubnov.[3] Among the Bolshevik revolutionaries who took part in the Russian Revolution of 1917, Stalin was appointed General Secretary of the party's Central Committee in 1922. He subsequently managed to consolidate power following the 1924 death of Vladimir Lenin by suppressing Lenin's criticisms (in the postscript of his testament) and expanding the functions of his role, all the while eliminating any opposition. He remained General Secretary until the post was abolished in 1952, concurrently serving as the Premier of the Soviet Union from 1941 onward.

Under Stalin's rule, the concept of "Socialism in One Country" became a central tenet of Soviet society, contrary to Leon Trotsky's view that socialism must be spread through continuous international revolutions. He replaced the New Economic Policy introduced by Lenin in the early 1920s with a highly centralised command economy, launching a period of industrialization and collectivization that resulted in the rapid transformation of the USSR from an agrarian society into an industrial power.[4] However, the economic changes coincided with the imprisonment of millions of people in Gulag labour camps.[5] The initial upheaval in agriculture disrupted food production and contributed to the catastrophic Soviet famine of 1932–33, known in Ukraine as the Holodomor. Between 1934 and 1939 he organized and led a massive purge (known as "Great Purge") of the party, government, armed forces and intelligentsia, in which millions of so-called "enemies of the working class" were imprisoned, exiled or executed, often without due process. Major figures in the Communist Party and government, and many Red Army high commanders, were killed after being convicted of treason in show trials.[6]

In August 1939, after failed attempts to conclude anti-Hitler pacts with other major European powers, Stalin entered into a non-aggression pact with Nazi Germany that divided their influence and territory within Eastern Europe, resulting in their invasion of Poland in September of that year, but Germany later violated the agreement and launched a massive invasion of the Soviet Union in June 1941. Despite heavy human and territorial losses, Soviet forces managed to halt the Nazi incursion after the decisive Battles of Moscow and Stalingrad. After defeating the Axis powers on the Eastern Front, the Red Army captured Berlin in May 1945, effectively ending the war in Europe for the Allies.[7][8] The Soviet Union subsequently emerged as one of two recognized world superpowers, the other being the United States.[9] Communist governments loyal to the Soviet Union were established in most countries freed from German occupation by the Red Army, which later constituted the Eastern Bloc. Stalin also had close relations with Mao Zedong in China and Kim Il-sung in North Korea.

Stalin led the Soviet Union through its post-war reconstruction phase, which saw a significant rise in tension with the Western world that would later be known as the Cold War. During this period, the USSR became the second country in the world to successfully develop a nuclear weapon, as well as launching the Great Plan for the Transformation of Nature in response to another widespread famine and the Great Construction Projects of Communism. In the years following his death, Stalin and his regime have been condemned on numerous occasions, most notably in 1956 when his successor Nikita Khrushchev denounced his legacy and initiated a process of de-Stalinization. Stalin remains a controversial figure today, with many regarding him as a tyrant.[10] However, popular opinion within the Russian Federation is mixed.[11][12][13] The exact number of deaths caused by Stalin's regime is still a subject of debate, but it is widely agreed to be in the order of millions.