In September 1936, a group of ten affiliated unions calling itself the Committee for Industrial Organization (CIO) was ejected from the American Federation of Labor (AFL) by vote of the executive council. Led by John L. Lewis and the United Mineworkers, the CIO would go on to become the most powerful labor organization in US history, attaining a level of national clout never matched before or since its heyday.

During the 1930s, the CIO rolled through the industrial heartland of the United States like a juggernaut, inspiring millions of workers to wage thousands of strikes and organize themselves into permanent local unions. CIO-affiliated militants, spread across the bastions of basic industry, worked together to build a system of industrial pattern bargaining that set wages and working conditions for millions of US workers (even beyond the ranks of the labor movement) and could be called upon to shut down entire sectors of American capitalism.

Though its role is rarely mentioned, and even less frequently assessed, the Left was central to the organization of this new dissident union federation. Thousands of socialist activists participated in the small and large battles that built the CIO over the two decades of its existence, and their victories and mistakes profoundly shaped the union federation’s history.