 The discussion of ad bellum justice concerns the fundamentals of just war theory, including: the moral permissibility of war; the difference between defensive and offensive war; legitimate authority to declare war; the requirement of a reasonable hope of victory; the constraints imposed by charity in any resort to war; and the moral implications of the domestic costs of wars which are in principle just. It also contains a compelling discussion of the duty of participants in war to investigate the presence of a just cause, as it applies to decision-makers, advisors, and various types of combatants, including mercenaries.