A united front is an alliance of groups against their common enemies, figuratively evoking unification of previously separate geographic fronts or unification of previously separate armies into a front. The name often refers to a political and/or military struggle carried out by volunteers or revolutionaries, including revolutionary socialism, communism, or anarchism, among others. The basic theory of the united front tactic among socialists was first developed by the Communist International, an international communist organization created by communists in the wake of the October Revolution. According to the thesis of the 1922 4th World Congress of the Communist International:

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.

According to Russian communist Leon Trotsky, the roots of the united front go back to the practice of the Bolshevik Party during the 1917 Russian Revolution. The Communist International generalized that experience among the fledgling communist parties that were established or grew significantly during the years after 1917. The theory of the united front was elaborated at the 3rd and the 4th Congresses of the Communist International, 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 strategy was used by leaders after the initial revolutionary tide since 1917 began to ebb. According to the leaders of the Communist International, 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 those leaders demanding unity on the basis of a united front. That would pose a dilemma for the reformist leaders: to refuse the invitation and be seen by their followers as an obstacle to unity or to accept the invitation and be required to operate on the terrain of mass struggle (strikes, protests etc.) on which the revolutionaries would be proved to have superior ideas and methods.