The history of the vexed relationship between clergy and warfare is traced through a careful examination of canon law. In the first millennium the Christian Church forbade its clergy from bearing arms. In the mid-eleventh century the ban was reiterated many times at the highest levels: all participants in the battle of Hastings, for example, who had drawn blood were required to do public penance. Yet over the next two hundred years the canon law of the Latin Church changed significantly: the pope and bishops came to authorize and direct wars; military-religious orders, beginning with the Templars, emerged to defend the faithful and the Faith; and individual clerics were allowed to bear arms for defensive purposes. This study examines how these changes developed, ranging widely across Europe and taking the story right up to the present day; it also considers the reasons why the original prohibition has never been restored. Lawrence G. Duggan is Professor of History at the University of Delaware and research fellowof the Alexander von Humboldt Foundation.