Trotskyism is an extreme left-wing political ideology which originated in the split in the Communist Party of the Soviet Union in the 1920s between supporters of Stalin and supporters of Leon Trotsky. Although personal animosity between Trotsky and Stalin was a major factor in the split, it came to be seen as centred on the incompatibility of views regarding the 'Revolution'. Trotsky believed that although the 'working class' had seized power in Russia, true socialism could not be established unless there was a global revolution. The Stalinists denied this, and held that they could create a socialist society in the USSR: the so-called 'Socialism in One Country' strategy. A wilier political operator than his opponent, Stalin was able to edge Trotsky and his supporters out of political power in the Soviet Union, and, by achieving domination of the Third International (the Comintern), was able to impose anti-Trotskist views on the worldwide Communist movement. Trotsky was exiled from the USSR in 1929, and murdered by Stalinist agents in Mexico in 1940.
Trotsky initially sought to recapture communist parties around the world from Stalinist influence, but inevitably small dissident groups, expelled from 'mainstream' communist parties, began to form independent organisations, often practicing entrism in non-communist left-wing parties in an attempt to take them over. In 1938 Trotsky formed the Fourth International, an umbrella body for Trotskyist national groups which sought to provide a global leadership.