One of the chief ways in which states have tried to order, that is to limit, the violence of war is through law. The international law of war grew out of customary practices developed by armies as they clashed on the field. There were three great periods when these customs were codified into written law: the seventeenth century (done by individual writers, most notably Hugo Grotius), the late nineteenth/twentieth centuries (done by international conferences), and the late twentieth/twenty-first centuries (done by international conferences, judicial extension, the establishment of international courts, and of the International Court under international auspices). This essay examines the second period, when a fundamental disagreement occurred between Imperial Germany and most other Western states over whether war could be limited at all. That disagreement, which had profound effects on how war was actually prosecuted, hinged on the definition of military necessity.

The term “military necessity” is a technical phrase of international law and custom coined to describe the spheres and circumstances in which lethal force or destruction may lawfully occur in time of war. Military necessity has a long history, yet it has been surprisingly little studied. In the late nineteenth and early twentieth centuries it was especially controversial because, as nations codified custom into written law, they successively narrowed the meaning of military necessity. Imperial Germany battled against this development. It claimed wide latitude for military necessity – so wide as to cancel out law altogether, contemporary observers from other countries charged.