Complex sentence: That same year, several Comintern affiliate organizations were established: the American League Against War and Fascism, followed by the American Youth Congress in 1934; the League of American Writers in 1935; and the National Negro Congress in 1936.
Split into simpler sentences: That same year however, a Comintern affiliate organization, the American League Against War and Fascism, had already been established. Followed by the American Youth Congress in 1934; the League of American Writers in 1935; the National Negro Congress in 1936.