Abstract: Warfare has become a legal institution. Law organizes and disciplines the military, defines the battlespace, privileges killing the enemy, and offers a common language to debate the legitimacy of waging war – down to the tactics of particular battle. At the same time, law is no longer a matter of firm distinctions –combatant and non-combatant, war and peace. It has become a flexible and strategic partner for both the military and for humanitarians seeking to restrain the violence of warfare. The relationship between modern war and modern law is made all the more complex by today’s asymmetric conflicts, and by the loss of a shared vision about what the law means and how it should be applied. Nevertheless, when law works well, it can be a strategic ally and provide a framework for talking across cultures about the justice and efficacy of wartime violence. When it works poorly, all parties feel their cause is just and no one feels responsible for the deaths and suffering of war. Kennedy explores the ways in which good legal arguments can make people lose their moral compass and sense of responsibility for the violence of war.