The Communist League of America (Left Opposition) was founded by James P. Cannon, Max Shachtman and Martin Abern in 1928 after their expulsion from the Communist Party USA for Trotskyism. The CLA (LO) was the United States section of Leon Trotsky's International Left Opposition and initially positioned itself as not a rival party to the CPUSA but as a faction of it and the Comintern. The group immediately began publication of The Militant as its regular newspaper.

The group soon became known simply as the Communist League of America. As the CPUSA and the Comintern became increasingly Stalinized the tactic of acting as an external faction of the Communist Party was replaced with plans to create the Fourth International as a new revolutionary international to replace the Third International and to replace the Communist Party with a new mass workers party.

In 1934, the CLA merged with A. J. Muste's American Workers Party to form the Workers Party of America. In 1936, the new party negotiated a fusion with the Socialist Party of America, however, the CLA continued to exist as an independent tendency and continued publishing their own newspaper, Socialist Appeal

The Trotskyists success in recruiting a large number of members to the Socialist Party's youth wing, the Young People's Socialist League, concerned Norman Thomas to the degree that he decided to expel the CLA from the Socialist Party in 1937. However, the CLA was able to win most members of the YPSL and split them from the Socialist Party.

The enlarged group held a convention on January 1, 1938 to launch the CLA's successor, the Socialist Workers Party. Later that year the Fourth International was formally launched.