Complex sentence: The AFL had focused on organizing small craft unions since its founding in 1881 by Samuel Gompers, but at its 1935 convention, a caucus of industrial unions led by John L. Lewis formed the Committee of Industrial Organizations, the original CIO, within the AFL.
Split into simpler sentences: The AFL had focused on organizing craft unions since its founding in 1881 by Samuel Gompers. But at its 1935 convention, a caucus of industrial unions led by John L. Lewis formed the Committee for Industrial Organization, the original CIO, within the AFL.