Precursors
A generalized Left Wing had existed prior to 1919, but lacked organizational cohesion. The success of the Bolshevik Revolution in Russia and the end of World War I was an accelerant that made revolutionary socialism and important issue of the day for many in America and around the world.
One important forerunner of the organized Left Wing Section of 1919 was the magazine The Class Struggle, founded by Ludwig Lore of the New Yorker Volkszeitung. Lore's magazine, which first saw print in May 1917, related current events in Europe and discussed matters of import written by various adherents of the Zimmerwald Left with an eager English-speaking audience. Co-editing the magazine with Lore were Louis C. Fraina, a former member of the Socialist Labor Party and voluminous writer on themes relating to the European revolutionary movement, and Louis Boudin, a well known Marxist theoretician.
Another regular publication loyal to the left-wing was International Socialist Review published by Charles H. Kerr.
A Socialist Propaganda League of America had been formed in Boston and by late 1918 had succeeded in taking over the Boston local. The Boston newspaper, The Revolutionary Age became the major voice of the Left wing in late 1918 and early 1919.
Read more about this topic: Left Wing Section Of The Socialist Party, History