One of the fundamental questions of Just War Theory is whether the rules applying to armed conflict, ius in bello, can be separated entirely from the question of a just cause to go to war, ius ad bellum. It has been largely argued that Vattel''s treatise on the law of nations was among the first to introduce a separation of those two regimes, thus giving way to the evolution of a set of rules governing any conflict regardless of its cause as witnessed especially in the late 19th and 20th century. This paper argues that Vattel does in fact develop his argument on the basis of traditional Just War Theory and explores the interrelation of just causes to war and the formal rules that govern inter-state conflict as presented by Vattel. The paper shows that Vattel''s distinction between the formal rules of warfare and the law of just war is far from being absolute and argues that very similar lines of reasoning continue to inform today''s debate on the interrelation of these two legal regimes. While a strict separation of ius in bello and ius ad bellum is desirable from both a doctrinal and a pragmatic standpoint, the early example of Vattel and its striking resemblance to recent debate show the intrinsic difficulties of putting this dualism into practice.
