The Comintern, also known as the Third International or the Communist International, existed from 1919 to 1943; its seventh and last Congress was held in 1935. The Comintern was the center for helping Communist activities worldwide; it was theoretically in Moscow only because Russia was the first nation to come under Communist control. In Marxist-Leninist theory, it was inevitable that other nations would become Communist, so the Comintern looked at those national Communist parties as simply being local offices of the International. The extent to which national parties followed this varied by country, but, in the beginning, support was often enthusiastic for the one successful revolution. 
In practice, it was under Soviet government control as a way of funding Communist parties in other countries, and to ensure those parties' continued support of Soviet policy. Before Stalin consolidated power, however, that policy had a substantial world revolutionary component.
As Stalin consolidated, it continued operations, to the concern of various nations outside the Soviet Union. National parties increasingly condemned Nazi Germany while pushing for internal Communism. In 1935, Germany and Japan, later joined by Italy, signed the Anti-Comintern Pact. This lasted until denounced by Germany with the Molotov-Ribbentrop Pact of 1939.
In the Bolshevik Revolution, headed by Vladimir Lenin, there were factions focused on the world-revolution model of Leon Trotsky, as opposed to the control-Russia-first model of Josef Stalin. With Stalin's establishing control, the need for the Comintern diminished, and simply was not important to him in the middle of the Second World War.
Karl Marx had supported the "First International", as the International Working Men’s Association, which 1864 but lasting only a few years. It was followed, as the "Second International", by the Labor International launched in 1889, which, to Lenin, was moderate social democratic and not proletarian, so the Third was needed for purity. These movements considered themselves socialist, not communist.
In the First World War, temporary alliances between socialists and communists broke the issue of supporting the war, rather than revolution. This did not immediately create the Third International, and, indeed, there were discussions, after the war, of reviving the Second International. One such discussion was the Berne International Conference, described, by a Marxist source, of made up of "right-wing socialists", called social-chauvinists and centrists during the war. The Conference met in the Swiss capital, Bern, on February 3-10, 1919.
Hjalmar Branting, leader of the Swedish Socialists, who had supported the Russian Revolution, proposed a resolution to condemn the Bolshevik taking of power and creating what they called the "dictatorship of the proletariat", preferring democratic means for establishing socialism. Brantling also disapproved of Spartacist revolutionary approaches.  A resolution proposed by Brantling, opposing the Bolshevik approach and calling for democratic reforms, initially received a large number of votes. A different group argued against the proposal, saying there was not enough information, and the conference compromised on sending a study commission to Russia.
While agreeing to admit the commission, the Soviet government requested the admittance of the Soviet commission, which Lenin called "auditing dignitaries from Berne", to those countries whose representatives were on the Berne commission. Since the Berne group was not a governmental authority, it could not respond to that request. No commission of the Second International ever visited the Soviet Union.
Formation of the Third International
After the success of the Communists in Russia, they created a Communist International (Comintern), also called the Third International.
The first president of the Comintern, was Gregory Zinoviev, of the "left" faction, who headed the Comintern from 1919 to 1926. In 1927, he was associated with the Zinoviev Letter allegedly urging British Communists to revolt. He was not popular in the party, and eventually purged by Stalin as a Trotskyite. 
The position was then formally abolished, but actual control passed to Nikolai Bukharin, of the "right" faction, who lasted until 1928.
The first formal meeting was held in March 1919, in Moscow, "with 51 delegates present: 35 with decisive votes representing 17 countries; 16 with consultative votes representing 16 countries."; not all delegates were able to attend.
It created an Executive Committee of the Communist International (ECCI), made up of representatives from Russia, Germany, Austria, Hungary, the Balkan Federation, Switzerland and Scandinavia; and a 5-member Bureau elected by the ECCI.
Held from July 17 to August 7, 1920, initially in St. Petersburg but meeting in Moscow from July 23 onward. There was still a blockade of Russia, but 37 countries were represented by 218 delegates of whom 169 had decisive votes and 49 consultative votes. 
Lenin introduced his Draft Theses on National and Colonial Questions for the Second Congress of the Communist International,, in which he proposed a stage of a transitional national federation between independence from colonialism and final communism. Ho Chi Minh described the ideas in this document as his epiphany in politics, reconciling his nationalism with his belief in Communism.
Held in June-July 1921, the Third Congress dealt with the realities that revolutionary fervor in Europe had diminished, the Soviet Union controlled the Comintern, and the Soviet priority was their New Economic Policy. It continued the broad-front approach, and established a Red International of Trade Unions (Profintern) that may have reflected acceptance that world revolution would be not be quick.
By December, the ECCI formally called for a "'united front' of the proletariat that permitted limited cooperation with other socialist parties and trade unions but warned against capitulation to 'Centrist and semi-Centrist ideology.'"
Meeting in May 1922, the Fourth Congress continued to reflect both world-revolutionary and Soviet-focused viewpoints.
On the international side, major issues was a rephrasing of the term the meaning of what would come to be called the workers’ and peasants’ government. According to a Trotskyite analysis, this generalized the united front idea to include both local Comintern and reformist labor groups. Centralized Bolshevik control would be minimal. If successful, such a government would be a transition to the dictatorship of the proletariat, under Communist control. The "workers and peasants" term was seen as a useful phrase to reach the masses who were not committed Marxists. 
Convened in June 1924, the Fifth Comintern Conference had less effect on overall policy than other Congresses, as it was increasingly dominated by the split between Trotsky and the Soviet leadership.  There was a demand for "Bolshevization", or Comintern national parties restructured on the lines of the Russian Bolshevik Party, "permitting no fraction, tendencies or groups."  Those groups would report both to their national members, but also to the ECCI.
Joseph Stalin began active Comintern involvement in this Congress. While he continued to write about it until 1928, however, he had little direct participation, not attending the last two Congresses. 
The United Front tactic of cooperation with non-Communists was called, by Zinoviev, "the most debated question in our ranks". Trotsky did not support the idea of cooperation with social democrats.
Approaching the Second World War
Germany and Japan, later joined by Italy, were concerned enough to create the Anti-Comintern Pact, a forerunner of the Tripartite Pact. Comintern policy, in response, changed to forming "anti-fascist fronts".
As the Second World War approached, its members, after the signing of the Molotov-Ribbentrop Pact, changed from opposing Nazi Germany to opposing the "imperialist" allies of Germany. Of course, the Nazi invasion of the Soviet Union switched the Comintern line back against the Axis powers.
While the Comintern was to be dissolved in 1943, eventually replaced by the Cominform, less threatening to the Allies, Comintern-affiliated partisans were a core of underground resistance to Germany.
- Alexander Dallin and F. I. Firsov, ed. (2000), Dimitrov and Stalin 1934–1943: Letters from the Soviet Archives, Yale University Press, pp. 2-3
- Berne International Conference
- Laurence M. Larson (1919), The Socialistic Upheaval in Europe, in American Historical Association, National Board for Historical Service, National Council for the Social Studies, The Social Studies: the Historical Outlook, McKinley Publishing Co., p. 18
- Robert Conquest (1990), Chapter 5, "The Problem of Confession", The Great Terror: a Reassessment, Oxford University Press, pp. 84-104
- Leon Trotsky (1924), The First Five Years of the Communist International, vol. Volume I, Marxists Internet Archive, Editor's Note 1
- Trotsky, Editor Note 3
- V. I. Lenin (June 5, 1920), Draft Theses on National and Colonial Questions For The Second Congress Of The Communist International
- Lewis Siegelbaum, 1921: Comintern — The Third or "Communist" International, Seventeen Moments in Soviet History
- "Rearming Bolshevism: A Trotskyist Critique of Germany 1923 and the Comintern", Spartacist (no. English edition No. 56), Spring 2001
- Sophie Quinn-Judge (2002), Ho Chi Minh: The Missing Years, 1919-1941, University of California Press, ISBN 0520235339, pp. 55-
- Jane Degras, The Communist International, 1919-1943: Documents, Frank Cass: 1970, Volume II, p. 154, cited in Quinn-Judge, p. 57
- N. Steinmayr (September 2000.), Stalin and the Comintern, The Stalin Society, London