Fanaticism is usually seen as a deviant or extreme variant of an already irrational set of religious beliefs. Drawing a straight line from the Peasant Wars to Bolshevism, this view of fanaticism is today invoked by the West in order to demonize and psychologize any non-liberal politics. Alberto Toscano's compelling counter-history explores the critical role fanaticism played in forming modern politics and the liberal state, and undermines the idea that liberalism and fanaticism are irrevocably opposed. Tracing its development from the traumatic Peasants' War of early sixteenth-century Germany, to contemporary Islamism, Toscano tears apart the sterile opposition of ‘reasonableness' and fanaticism. Instead, in a radical new interpretation, he places the fanatic at the very heart of politics, arguing that historical and revolutionary transformations require a new understanding of its role. Showing how fanaticism results from the failure to formulate an adequate emancipatory politics, this illuminating history sheds new light on an idea that continues to dominate debates about faith and secularism.