The law of armed conflicts developed in an environment where recourse to war was licit. The right to resort to war was an attribute of sovereignty, and any sovereign who thought he had good reason to take up arms was free to do so. The legal environment is today totally different: the war of aggression has been outlawed by the Paris Pact of 27 August 1928 and the United Nations Chapter. The object of the present article is to analyse the relationship between the prohibition of recourse to force (“jus ad bellum”), on the one hand, and the application of the laws and customs of war (“jus in bello”), on the other. This issue is particularly pressing since the just war theory of the past is resurfacing today in various parts of the world.