The contribution focuses on the ways in which medieval Catholic religion influenced warfare, not in terms of causality but in terms of conditions of possibility. After having looked at (1) the way in which the crusades, in particular, opened up the possibility to transfer attributes from monastic asceticism (the monks were spiritual ‘warriors of God’) to warriors (fighting as ‘warriors of God’), the article examines two examples. (2) Theology provided to the Western European culture of war the figure of the ‘false brother’, which, translated, yielded a script for the internal enemy, the political traitor and adversary in civil wars. Around this figure enormous fantasies crystallized themselves, arguably without equivalent in non-monotheistic cultures. One sees in the late medieval French civil wars the semi-secularized mobilization of this figure, with fantasies and execution of violent purge. (3) The ascetic values activated for warfare with the First Crusade meant that while in reality (as one would expect) sexual did transgressions occur, rape in war by one’s own side was hardly ever admitted, and arguably was in reality also inhibited.