Book excerpt: The period immediately preceding World War II was probably the most critical in the history of the American labor movement. Prior to 1936, the trade unions were weak, but by 1941 a fundamental change in power relationships enabled them to penetrate the strongholds of American industry--steel and automobiles. The CIO Challenge to the AFL is a three-part study. It discusses the split in the American Federation of Labor and the formation of the Congress of Industrial Organizations; presents eighteen specific industry or union case studies, each an independent essay in economic history; and, finally, analyzes various general aspects of the labor movement.