Killing in war, according to one military psychologist, is ‘the single most basic, important, primal, and potentially traumatic occurrence of war’. Indeed, many individuals will risk death in order to avoid taking a human life. Yet war, and the killing which accompanies it, has been an ever-present phenomenon in human society, during which time it has been understood as a glorious and noble activity or condemned as an endemic disease resulting in pointless butchery: sometimes both at the same time. If the abovementioned psychological analysis is correct – that humans share an abhorrence of taking human life – this may partly explain why societies have attempted to justify, regulate and even restrain this peculiarly destructive and persistent human activity.

By the time that John Wyclif came, in the second half of the fourteenth century, to examine the problem of war, a well-established doctrine of justified violence had existed in the Christian West for a millennium. This doctrine was itself the product of a much older tradition. Ideas of justice and law regarding the resort to war, as well as norms regulating combat, evolved in the West from at least the first millennium bc, and were shaped according to the needs of significantly different societies and cultures. Religious ethics, as well as economic and political pragmatism, were powerful forces in shaping these ideas. However, the West was not unique in its reflections upon the ethical status of war and violent action. Ideas about the proper conduct of war are evident in the ancient Sumerian Epic of Gilgamesh (early second millennium bc), as well as the fifth-century bc Indian poem the Māhabhārata. The Chinese concept of yi bing, which understood war as the prerogative of the ruler and a tool of judicial punishment, was developed around the same time that Plato and Aristotle were writing in the Academy at Athens (c. 481–207 bc). Much later, particularly from the ninth century onwards, Islamic jurists developed a doctrine of jihad which sought to clarify when and why wars ought to be fought, as well as identifying who and what ought to be immune from military violence. Yet it was in the West that the ethical and legal analysis of war and violence enjoyed the most prolonged and dedicated attention.