A popular front is "any coalition of working-class and middle-class parties", including liberal and social democratic ones, "united for the defense of democratic forms" against "a presumed Fascist assault". More generally, it is "a coalition especially of leftist political parties against a common opponent".

The Communist International (Comintern), also known as the Third International, was an international organization founded in 1919 that advocated world communism, and which was led and controlled by the Communist Party of the Soviet Union. The Comintern resolved at its Second Congress in 1920 to "struggle by all available means, including armed force, for the overthrow of the international bourgeoisie and the creation of an international soviet republic as a transition stage to the complete abolition of the state". The Comintern was preceded by the dissolution of the Second International in 1916. Vladmir Lenin and Leon Trotsky were both honorary presidents of the Communist International.