Over the past forty years, after the Vietnam War, just war theorising, which had been largely ignored in mainstream reasoning about war and international relations under the hegemony of 'realism', began to reassert itself. Traditionally it was the province of theologically based reasoning deriving ultimately from authors such as Aquinas. However, in its modem incarnation (in which the lead was taken by Michael Walzer in his influential Just and Unjust Wars, originally published in 1977), it was recast as a secular theory, one not unrelated to its origins in the Catholic church, but conceptually independent of it and essentially the distillation of many centuries of reasoning on the justice and conduct of war. But we are wary of continuing to refer to it as 'a theory'. A better way to characterise it is as a tradition of moral reasoning. It is not a theory in the way that utilitarianism is a theory, both because it includes elements of both deontological and consequentialist reasoning and also because it does not seek to reduce all reasoning on war to a single explanatory principle. As George Lucas suggests, it is a 'mode of normative discourse, a philosophical tradition of reflection on the moral constraints of both prudential reasoning and political practice'. 1 Just war theory or tradition (henceforth JWT) should therefore be thought of both as an approach to moral reasoning about war (drawing on the accumulated wisdom of practical reasoning about war) and as a resource comprising the most salient conceptual and moral distinctions distilled over centuries of reasoning. It should not be thought of as an answergenerating machine operated by turning fact and value inputs into neatly minced outputs at the single tum of a handle. It is a way of reasoning, not a substitute for it. We return to these themes later. For the moment we will inspect some features of the history of JWT.