The permissive power of the ban on war

The ban on inter-state war in the UN Charter is widely identified as central to the modern international order-Michael Byers calls it 'one of the twentieth century's greatest achievements'. Even if it is only imperfectly observed, it is often seen as a constraint on state autonomy and an improvement on the pre-legal, unregulated world before 1945. In response to this conventional view, this article shows that the laws on war in the Charter are better seen as permissive rather than constraining. I make two points. First, by creating a legal category around 'self-defence', the laws on war authorise, and thus legitimate, wars that are motivated by the security needs of the state, while forbidding other motives for wars. Second, state practice since 1945 has expanded the scope of this authorisation, extending it in both time and space beyond the black-letter text of the Charter. The permissive effect of law on war has therefore been getting larger. These two effects suggest that international law is a resource that increases state power, at least for powerful states, and this relation between international law and power politics is missed by both realists and liberal internationalists.