This book looks at the evolving relationship between war and international law, examining the complex practical and legal dilemmas posed by the changing nature of war in the contemporary world, whether the traditional rules governing the onset and conduct of hostilities apply anymore, and how they might be adapted to new realities. War, always messy, has become even messier today, with the blurring of interstate, intrastate, and extrastate violence. How can the United States and other countries be expected to fight honourably and observe the existing norms when they often are up against an adversary who recognizes no such obligations? Indeed, how do we even know whether an "armed conflict" is underway when modern wars tend to lack neat beginnings and endings and seem geographically indeterminate, as well? What is the legality of anticipatory self-defense, humanitarian intervention, targeted killings, drones, detention of captured prisoners without POW status, and other controversial practices? These questions are explored through a review of the United Nations Charter, Geneva Conventions, and other regimes and how they have operated in recent conflicts. Through a series of case studies, including the U.S. war on terror and the wars in Afghanistan, Iraq, Gaza, Kosovo, and Congo, the author illustrates the challenges we face today in the ongoing effort to reduce war and, when it occurs, to make it more humane.