This article investigates the inter-play between jus ad bellum and jus in bello in order to see what effects as to substance and structure that the inter-play may have on each field. The contact between the twofields of law will probably lead to modifications in each. Contrary to what is often presumed, in the relative competition between jus ad bellum and jus in bello, it may be very well be that jus in bello will emerge as the strongest body of norms. This, moreover, would seem the most likely development given the current focus on the suffering of civilians during armed conflict and the rise of human rights and humanitarian concernsgenerally in international law. Jus in bello may come to condition jus ad bellum and not the otherway round. This may make the disappearing dichotomy between jus ad bellum and in bello seem less threatening. A strengthened jus in bello and related concerns may even come to push jus ad bellum into irrelevance or obsolescence.