 Vitoria and the other authors of the School of Salamanca dealt with martial law and developed a theory of just wars that was, on the one hand, based on Thomas Aquinas’ arguments and, on the other hand, built on the foundation of the “classical doctrine of just war” (Orend 2006). Vitoria dealt with just war in his lecture De Indis relectio posterior sive De iure belli. He discusses who can wage war—private persons only for self-defense, the commonwealth, and the prince also for other reasons (Vitoria 1991b: 299–302); what cannot be considered just reasons for waging war, such as difference in religion and enlargement of empire (1991b: 302–304); what can be done in a just war (1991b: 304–306, 314–317); and the view that it may not be enough if a prince believes that his war is just (1991b: 306–7). Vitoria regarded “perfect” commonwealths as able to defend themselves from internal and external threats. The right of self-defense permitted the use of force to repel force. A society, pagan or Christian, could wage a just war in self-defense and execute malefactors. Vitoria accepted repelling invaders and avenging injury as justifications for war, but not the invasion of realms in order to acquire land, slaves or plunder. Religious difference and the vainglory of princes were also not sufficient reasons. Those in authority could declare and wage war, but they should take counsel about this. Subjects could not refuse service in a just war. All necessary means were acceptable in a just conflict, even entering an enemy’s territory. Vitoria, however, applied a criterion of proportional harm to wars, so that a war was unjust if it caused more harm than good. Moreover, a ruler who had waged an unjust war was obliged to make restitution. Booty could recompense property unjustly seized by an enemy, but only if taken on a superior’s orders. Vitoria permitted enslaving Saracens defeated in a just conflict, but he denied that Christians could enslave other Christians. Moreover, he denounced devices used in capturing slaves for export. Overall, Christians were expected to abide by the rules of war, showing moderation and humility when victorious.