With the Second Gulf War, the use of arms acquired a constituent ambition: to build a different world order on the basis of the criterion of effectiveness. According to some realist approaches, there is no point in making nostalgic calls for international legality when faced with the effectiveness of preventive war. Law reveals its weakness vis-à-vis the will of states to exercise their power. Yet the disenchantment of the realist who is seduced by the principle of effectiveness neglects the fact that, in an increasingly interdependent world space, it is advisable to have an agreed legal framework. A less legal world does not become safer by any means. The logic of law and of the international institutions therefore comes back into play after the crude grammar of naked force as the most realistic solution.