James P. Cannon (1890-1974) helped build the American revolutionary left. Reared in a radical Midwestern household, he served a class struggle apprenticeship in the Socialist Party, the Industrial Workers of the World, and the underground communist movement. From the founding of the Workers Party in 1921, Cannon guided the forces of United States communism. Increasingly disappointed in the international and domestic leadership of the revolutionary movement, Cannon eventually embraced Trotsky's criticisms of emerging Stalinism. When he was expelled from the Workers (Communist) Party in 1928, a particular age of U.S. radicalism had come to an end, but another was just beginning. Bryan D. Palmer's magisterial study is both a biographical treatment of Cannon's formative years as well as a richly detailed and passionately argued examination of a pivotal epoch of American radicalism. Meticulously and imaginatively researched, it brings to life a major figure in the underappreciated United States revolutionary tradition. It also recasts our understanding of those movements Cannon championed, from the Wobblies and Left-Wing of the Socialist Party to early communism and its decline under Stalinization.