International law and armed conflict have a rather contentious history together. One the one hand, armed conflict implies and absence of law, and yet, on the other, international law plays an important role in codifying the use of force. The UN Charter’s restrictions on the use of force, drafted in the waning days of a second cataclysmic world war, were intended to radically transform the centuries-old ideology of raison d’état, which viewed war as a sovereign prerogative. More precisely, Article 2(4) of the Charter forbids not just war but force of any kind, or even the threat of it. On its face, the Charter system is a model of simplicity, consisting of a clear prohibition and two exceptions to that prohibition. The apparent simplicity is misleading, however. Article 2(4) is violated so often that experts disagree about whether it should even be considered good law. The Chapter VII enforcement exception is rarely used, and the meaning of self-defense under Article 51 is the subject of contentious disagreement. Moreover, even some UN bodies have supported creating another exception (humanitarian intervention) that coexists uneasily with the organization’s foundational principles. In addition, there is yet another exception (the use of force by national liberation movements) that may be as significant as the others, yet is little discussed by contemporary commentators.