Uniquely, this book traces the decisive political divisions within the broadly-defined Trotskyist movement by presenting key texts from both sides of the political debate as it happend. The book’s overall thesis, argued in a substantial introduction, is that by the late 1940s there were two Trotskyisms. They had separated, fundamentally, through their different responses to events neither “side” expected: the transition of the Stalinist USSR from unstable, beleaguered semi-outlaw state to a continent-bestriding world power, stably self-reproducing at least for some decades to come. The other issues were many, but, so the book argues, mostly linked to that fundamental division. They included different conceptions of what a revolutionary party should be and do, and what Marxism is and how it is developed. The once-dominant “orthodox” variant of Trotskyism has been in disarray since the collapse of European Stalinism in 1989-91. Today’s revolutionary socialist politics, struggle to regain ground after the long triumph of neoliberalism, needs to nourish itself by studying these long-shelved debates at the hinge of 20th century politics.