International history and International Relations have long been held separate, partly by misunderstanding and partly by mistrust. Three recent books, Marc Trachtenberg's Craft of international history, Paul Kennedy's The parliament of man and Niall Ferguson's The war of the world, suggest that the divide between history and theory is not as severe as it sometimes appears. This review article examines, through the histories of Kennedy and Ferguson, Trachtenberg's insistence that historians should be more attentive to the 'conceptual cores' of their work and that theorists should become better historians than they have been hitherto. It concludes by arguing that, in methodological terms at least, history and theory are not the distinct enterprises they are commonly taken to be.