This thesis uses an ex-communist network to offer a new perspective on interwar Marxism. I focus on a network that formed in the early 1930s, composed of intellectuals that defected from the international communist movement in the 1920s. Some of its principal figures are Karl Korsch, Arthur Rosenberg, Boris Souvarine, Franz Borkenau and Lucien Laurat. On a practical level, the network in question was transnational, communicating across Western Europe and, occasionally, beyond into North America and the USSR, reflecting something of Marxism as a transnational community. I argue that interwar ex-communist Marxists were torn between the apparent success of revolutionary Marxism and its equally apparent shortcomings. The narrative traces this dialectic with respect to (1) reflexive Marxist theory, (2) the place of revolution in history, and (3) narratives of political economy. It was not defeat by the Nazis or the degeneration of the Russian Revolution, as much of the literature assumes, but the failure of these failures to make sense, that drove the dialectic of disillusionment. At the same time, other factors worked in the opposite direction, preventing a total break. Didn’t the coming of the Second World War confirm Lenin in his forecast of an epoch of wars and revolutions? Wasn’t the Great Depression a final crisis of capitalism? Wasn’t Marxism, anyway, nothing more than a method that could be applied to anything history threw up? The period was marked by a deniable plausibility for the basic shape of revolutionary Marxism that found it greatest confirmation and final disappointment in the Second World War—a long-expected repeat of the First, and at the same time essentially different. The ground had been prepared over the course of the 1930s, but it was this disappointment that closed this chapter of Marxism’s history and set the interwar moment apart from its successors.