The Second World War acted like an earthquake on the international law of war and left some of it in ruins. This essay attempts to explain why. But before analysing the nature of the historic forces, and, so far as they are relevant, specific events which produced this disturbance, it will be helpful if we remind ourselves of something even more basic: the intrinsic fragility of the law of war – a fragility which exists and has its unsettling effects even when historic forces happen to be helpful. The law of war of course shares in the chronic weakness of international law in general, that is it is a law without a sovereign – rules without a ruler. But before that, the law of war rests uniquely upon a paradox which hostile critics have not hesitated to call an absurdity. It offers mediation between the demands of on the one hand Humanity, on the other Military Necessity. It opens a middle path between the peaceable brotherly conduct which civilized men believe they would, in favourable circumstances, like to observe towards their neighbours, and the lethal violence they find themselves using when the circumstances are those of war. It requires a cohabitation of logical inconsistencies which some – certainly not all – interpretations of Humanity or Military Necessity have alleged to have been no more practical than squaring the circle. Yet the idea of the law of war has survived, something about which the philosopher and psychologist at any rate might have much to say. The historian, however, feels obliged to pass by those deeper levels of his theme with the reflection, for what it is worth, that such tolerance, even appetite, for inconsistency need not surprise anyone whose idea of human nature includes a divided mind or whose idea of the human condition includes tragedy.