This is a history of the phenomenon of war, as viewed through the lens of international law. There is, to be sure, no such thing, strictly speaking, as the phenomenon of war, majestically constant throughout history and across the various human cultures. War, like other human practices, has always been a protean thing, incessantly changing its face throughout the course of recorded history in response to a dizzying array of factors – religious, technological, economic, psychological, political and so forth. And its history has been duly analysed from many of these standpoints. But the perspective of international law has been strangely neglected. Some attention (but surprisingly little) has been devoted to the history of the development of rules governing the conduct of war. Our concern, however, is different: it is with the deeper ideas about the legal nature of war itself and how those have changed over the course of human history. This is, in short, a history of the way in which fundamental legal conceptions of war have evolved from the most distant retrievable past to the present day.

Much of our current picture of war is coloured by images of nineteenth-century conflicts between European states. This stereotype calls to mind solemnly proclaimed declarations and the summoning of ranks of uniformed troops (sometimes rather gaudily uniformed at that), in orderly arrays. These forces then engaged in combat on a field of battle against forces similarly decked out.