The Communist International, abbreviated as Comintern and also known as the Third International, was a political international which existed from 1919 to 1943 and advocated world communism. Emerging from the collapse of the Second International during World War I, the Comintern was founded in March 1919 at a congress in Moscow convened by Vladimir Lenin and the Russian Communist Party (Bolsheviks) (RCP), which aimed to create a new international body committed to revolutionary socialism and the overthrow of capitalism worldwide.

Initially, the Comintern operated with the expectation of imminent proletarian revolutions in Europe, particularly Germany, which were seen as crucial for the survival and success of the Russian Revolution. Its early years were characterized by attempts to foment and coordinate revolutionary uprisings and the establishment of disciplined communist parties across the globe, often demanding strict adherence to the "Twenty-one Conditions" for admission. As these revolutionary hopes faded by the early 1920s, the Comintern's policies shifted, notably with the adoption of the "united workers' front" tactic, aiming to win over the working masses from reformist socialist parties. Throughout the 1920s, the Comintern underwent a process of "Bolshevisation", increasing the centralization of its structure and the dominance of the RCP within its ranks. This process intensified with the rise of Joseph Stalin in the Soviet Union.

The "Third Period" (1928–1933) saw the Comintern adopt an ultra-sectarian line and denounce social democratic parties as "social fascism". From 1934, the Comintern shifted to the Popular Front policy, advocating broad alliances with socialist and even liberal parties against fascism. This was formally adopted at its Seventh World Congress in 1935. The Comintern played a significant role in organizing support for the Republican side in the Spanish Civil War, including the formation of the International Brigades. However, this period also coincided with the Great Purge in the Soviet Union, during which many Comintern officials and foreign communists residing in Moscow were arrested and executed.

With the signing of the Nazi–Soviet Pact in August 1939, the Comintern again changed its line, denouncing the war between Nazi Germany and the Western Allies as an "imperialist war" and abandoning its anti-fascist stance until the German invasion of the Soviet Union in June 1941. As a gesture to its Western Allies in World War II, Stalin unilaterally dissolved the Comintern on 15 May 1943. While its formal structures were dismantled, mechanisms of Soviet control over the international communist movement persisted and were later partially revived through the Cominform (1947–1956).

The Comintern, or Third International, was the direct descendant of the First International (1864–1872) and the Second International (1889–1916). The First International, of which Karl Marx was a leading figure, aimed to coordinate the proletariat in its worldwide struggle against capitalism, based on the premise that "the working men have no country" and that horizontal class allegiance would supersede vertical national divisions. By the late 19th century, however, the Western labour movement had largely abandoned the revolutionary zeal of the First International. Powerful trade unions and socialist parties emerged which, while often adhering to Marxist revolutionary theory, in practice pursued gradual, constitutional reforms, improving the workers' lot within the existing capitalist framework. This created a manifest contradiction between revolutionary rhetoric and peaceful practice.

The Second International, created in 1889, was a looser federation of autonomous socialist parties, comprising "left", "right", and "centrist" factions divided on issues such as bourgeois democracy, the national question, general strikes, and, crucially, war. It foundered in August 1914 on the rock of national chauvinism when most of its constituent parties chose to support their respective national governments in World War I by voting for war credits. Vladimir Lenin, a key figure in the Russian Bolshevik Party (later the Russian Communist Party, RCP), viewed this as a "sheer betrayal of socialism" and declared the Second International dead, calling for a Third International by the autumn of 1914.

The Bolsheviks emerged from a distinct Russian revolutionary tradition. Unlike the mass-based, reformist parties of the West, Russian revolutionaries operated in a clandestine underground, organized in small, disciplined groups of "professional revolutionaries". Drawn largely from the intelligentsia, this movement possessed a quasi-religious devotion to the cause of revolution itself, an enthusiasm that contrasted sharply with the practical, matter-of-fact nature of Western labour movements. During the war, a minority of intransigent revolutionaries coalesced around the Bolsheviks, forming the Zimmerwald Left. Lenin advocated turning the international war into a revolutionary civil war and was implacably hostile to parliamentary democracy. The October Revolution of 1917 in Russia, led by the Bolsheviks, was seen by Lenin as the first act of a global drama, with European workers expected to follow suit. To achieve this, a new International, purged of reformist "traitors", was deemed an absolute necessity.

The Communist International was founded at a congress of revolutionaries in Moscow from 2–6 March 1919. The impetus for its creation came from the Bolsheviks' belief in the imminence of world proletarian revolution, spurred by the perceived collapse of capitalism after World War I and revolutionary upheavals across Europe, particularly the German "November Revolution". The mission of the Comintern was to build a "world party" of communists dedicated to the armed overthrow of capitalist private property and its replacement by a system of collective ownership.