The face of modern warfare is changing as more and more humanitarian organizations, private military companies, and non-state groups enter complex security environments such as Iraq, Afghanistan, and Haiti. Although this shift has been overshadowed by legal issues connected to the War on Terror and intervention in countries such as Rwanda and Sudan, it has caused some to question the relevance of the laws of war. To bridge the widening gap between the theory and practice of the law, Modern Warfare brings together both scholars and practitioners who offer diverse perspectives on the key challenges to the law’s legitimacy: how to ensure compliance among non-state groups; the proliferation of private military companies in the employ of humanitarian organizations; tension between the idea of humanitarian space and counterinsurgency doctrines; and the phenomenon of urban violence. By bringing to light the law’s limitations – and potential – this timely book opens a path to preventing further unnecessary suffering and violence.