In its most basic terms, war depends upon getting men to risk their bodies, to override the instinct to flee danger so as to preserve life, and instead to stand and fight, and if necessary, to fight to the death. The means by which this is achieved can be understood variously as the effects of the noble virtue of courage at one end of the spectrum, and the despicable hardening of men into beasts at the other. The Middle Ages has not shaken off its reputation as a violent, conflict-ridden age (a view not without some justification), nor has it put to rest the charge that its codes of chivalry existed to keep violent men in check. Yet learned, Latinate treatises in this period (on natural philosophy, and on governance, among other subjects) confront the difficult ethics of war, and how it is men are brought to engage in battle and sacrifice their lives in its cause. The answers these treatises offer draw on complex understandings of human physiology and psychology. Towards the end of the fourteenth century and into the fifteenth, some such treatises, like Giles of Rome's widely influential De regimine principum, are translated into vernacular languages or are drawn on by vernacular authors writing on war and chivalry. This essay focuses on these learned ideas about physiology and psychology as they appear in a set of late medieval English writings on war, principally those translating or reworking (in whole or in part) Giles's De regimine principum and Vegetius’ De re militari.