Jus ad Bellum

International law shares with the Just War tradition many core ideas. But the most important is that war should not be initiated unless there is a just cause, principally understood in terms of self-defense. And in international law, as in the Just War tradition, self-defense is narrowly circumscribed by considerations of necessity and proportionality, among others. Jus ad bellum primarily concerns the use of force in initiating armed conflict. In this chapter I will explore the main contours of the idea of justified war in international law and also draw contrasts with the Just War considerations we have already examined in previous chapters. There are significant areas of overlap and mutual influence between jus ad bellum international law and Just War theory today. Self-defense is the most widely recognized justification for engaging in war. Indeed, there are great controversies about whether anything other than self-defense can justify recourse to war. As we will see, international court decisions have interpreted the UN Charter Article 2, section 4's broad prohibition on the use of force as having a major exception in Article 51's articulation of an inherent right of individual or collective self-defense. As I will explain, self-defense exceptions to the prohibition on the use of force by states against other states are severely limited. But there is wide agreement in international law that at least until the UN has acted States have the right to use lethal force in self-defense. The main international law texts on the use of force are Article 2, section 4, and Article 51 of the Charter of the United Nations. I will begin by examining these texts and then turn to case law and commentary that have refined understanding of the principles governing the use of force. So, in Section I, I provide an analysis of Article 2, section 4, and Article 51. In Section II, I examine a case that has had profound implications for the understanding of the use of force, that of Nicaragua. In Section III, I look at two other cases that have been highly influential. In Section IV, I examine how individuals have been treated in international criminal law when jus ad bellum norms have been violated. And finally, I draw out some of the parallels between debates about the use of force in international law and in the Just War tradition.