A united front is an alliance of groups against their common enemies, figuratively evoking unification of previously separate geographic fronts and/or unification of previously separate armies into a front (formation); the name often refers to a political and/or military struggle carried out by revolutionaries, especially in revolutionary socialism, communism or anarchism. The basic theory of the united front tactic among socialists was first developed by the Comintern, an international communist organization created by communists in the wake of the 1917 Bolshevik Revolution.
According to the thesis of the 1922 4th World Congress of the Comintern:
“The united front tactic is simply an initiative whereby the Communists propose to join with all workers belonging to other parties and groups and all unaligned workers in a common struggle to defend the immediate, basic interests of the working class against the bourgeoisie.”
The united front allowed workers committed to the revolutionary overthrow of capitalism to struggle alongside non-revolutionary workers. Through these common struggles, revolutionaries sought to win other workers to revolutionary socialism. The united front perspective is also used in contemporary and non-Leninist perspectives.
According to Russian activist Leon Trotsky, the roots of the united front go back to the practice of the Bolshevik Party in the 1917 Russian Revolution. The Comintern generalised this experience among the fledgling Communist parties that were established or grew significantly during the years following 1917. The theory of the united front was elaborated at the third and fourth congresses of the Comintern, held from November 5 to December 5, 1922.
Revolutionary socialists represented a minority in the working class, and the united front offered a method of working with large numbers of non-revolutionary workers and simultaneously winning them to revolutionary politics. The united front strategy was used by leaders in the period after the initial revolutionary tide following 1917 began to ebb. According to leaders of the Comintern, the shift from offensive to defensive struggles by workers strengthened the desire for united action within the working class. The leaders hoped that the united front would allow the revolutionaries to win a majority inside the class:
"The task of the Communist Party is to lead the proletarian revolution. In order to summon the proletariat for the direct conquest of power and to achieve it the Communist Party must base itself on the overwhelming majority of the working class.... So long as it does not hold this majority, the party must fight to win it."
The revolutionaries were told to maintain independence:
"The existence of independent Communist Parties and their complete freedom of action in relation to the bourgeoisie and counter-revolutionary social democracy.... In the same way the united front tactic has nothing to do with the so-called 'electoral combinations' of leaders in pursuit of one or another parliamentary aim. The united front tactic is simply an initiative whereby the communists propose to join with all workers belonging to other parties and groups and all unaligned workers in a common struggle to defend the immediate, basic interests of the working class against the bourgeoisie."
However, revolutionaries could not simply go over the heads of the leaders of reformist organizations. They should approach these leaders demanding unity on the bases of a united front. This would pose a dilemma for the reformist leaders: refuse the invitation and be seen by their followers as an obstacle to unity, or accept the invitation and have to operate on the terrain of mass struggle (strikes, protests, etc.) on which the revolutionaries would be proved to have superior ideas and methods.
As Stalinism came to dominate the Comintern, the united front strategy was dropped. In the period preceding Adolf Hitler's electoral victory in Germany, the Comintern argued that the social democrats were "social fascists" and that they, rather than the Nazis, represented the real danger. Following Hitler's victory, the Comintern argued for popular fronts drawing in forces far beyond the working class movement. Trotsky, now exiled from the USSR, argued that the first conclusion was disastrous because it prevented unity against the far right, and that the second, emphasizing popular fronts, was disastrous because the terms of the struggle would be dictated by mainstream liberal parties. He feared that the communists would have to subordinate their politics within the alliance. Trotsky continued to argue for a workers' united front against fascism.
Trotsky argued that the united front could have great appeal to workers who wished to fight fascism:
"The programme of action must be strictly practical, strictly objective, to the point, without any of those artificial 'claims', without any reservations, so that every average Social Democratic worker can say to himself: what the Communists propose is completely indispensable for the struggle against fascism. On this basis we must pull the Social Democratic workers along with us by our example, and criticize their leaders who will inevitably serve as a check and a break.” 
In Chinese history, the First United Front (1924–27) was a period when the Communists worked closely with the Kuomintang. The Chinese organized a Second United Front (1937–43) to fight the Japanese during World War II.
In Vietnam, the Vietcong organized the National Liberation Front (1960–77) to gather widespread support for the independence struggle, first against France and later against the United States during the Vietnam War. Trotsky and Trotskyists, such as Harold Isaacs in his The Tragedy of the Chinese Revolution, would argue that these were popular fronts, not united fronts, based upon the model used by the Bolsheviks in 1917 and afterwards.
- "Theses on Comintern Tactics". 1922. Retrieved 2008-02-20..
- Dan Jakopovich, "In the Belly of the Beast: Challenging US Imperialism and the Politics of the Offensive", CC-DS
- Leon Trotsky (1931). "What next? Vital Questions for the German Proletariat". Retrieved 2008-01-07..
- Leon Trotsky (1924). "General Considerations of the United Front". Retrieved 2008-02-20..
- Joseph Choonara (2007). "The United Front". International Socialism Journal. Retrieved 2008-02-20.
- See also Florian Wilde: Building a Mass Party: Ernst Meyer and the United Front Policy 1921-1922, in: Ralf Hoffrogge / Norman LaPorte (eds.): Weimar Communism as Mass Movement 1918-1933, London: Lawrence & Wishart 2017.
- Leon Trotsky. "The Rise of Hitler and the Destruction of the German Left". Retrieved 2008-01-07.
- Leon Trotsky (1931). "the Workers United Front Against Fascism". Marxists Internet Archive. Retrieved 2008-02-20..
- Ang, Cheng Guan (2002). The Vietnam War from the Other Side. RoutledgeCurzon. p. 55. ISBN 0700716157.
- Harold Isaacs. "The Tragedy of the Chinese Revolution". Marxist Internet Archive. Retrieved 2009-08-28.
- Florian Wilde: Building a Mass Party: Ernst Meyer and the United Front Policy 1921-1922, in: Ralf Hoffrogge / Norman LaPorte (eds.): Weimar Communism as Mass Movement 1918-1933, London: Lawrence & Wishart 2017. pp. 66-86.
- The United Front by Joseph Choonara in International Socialism 117 (2007)
- On the United Front tactic by Duncan Hallas in International Socialism (1976)
- The United Front by Lindsey German (1984)
- The United Front by Pete Goodwin in International Socialism (1978)
- Lewis Mates (2002). "The United Front and the Popular Front in the North-east of England, 1936-1939" (Thesis).