The just war tradition has been the normative Christian method of moral reasoning about the responsible use of armed force in world politics by legitimate public authorities for some fifteen hundred years. Over a millennium and a half, and even as its central ideas have endured, the tradition has developed in response to political and technological innovation. In times like our own, which feature rapid political and technological change, the development of the just war way of thinking must, of necessity, accelerate. The purpose of this essay is to identify the changes in world politics and in weapons technology that require a development of the tradition in the early twenty-first century; to note the obstacles to that development posed by certain defective understandings of the just war tradition; to suggest several areas in which the tradition needs development, while proposing lines along which that development might take place; and to ask whether one might see such a developed tradition shaping and indeed being unfolded in US national security policy.

While the roots of the just war tradition reach back to Greek and Roman political theory, a specifically Christian idea of the just war was first enunciated by Augustine in The City of God, and was only later systematised: canonically by Gratian and his successors, the Decretists and the Decretalists; theologically, by Thomas Aquinas.