This volume combines philosophical analysis with normative legal theory. Although both disciplines have spent the past fifty years investigating the nature of the principles of necessity and proportionality, these discussions were all too often walled off from each other. However, the boundaries of these disciplinary conversations have recently broken down, and this volume continues the cross-disciplinary effort by bringing together philosophers concerned with the real-world military implications of their theories and legal scholars who frequently build doctrinal arguments from first principles, many of which herald from the historical just war tradition or from the contemporary just war literature. What unites the chapters into a singular conversation is their common skepticism regarding whether the traditional doctrines, in both law and philosophy, have correctly valued the lives of civilians and combatants at war. The arguments outlined in this volume reveal a set of principles, including necessity and proportionality, whose core essence remains essentially contested. What does military necessity mean and are soldiers always subject to lethal force? What is proportionality and how should military commanders attach a value to a military target and weigh it against collateral damage? Do these valuations remain the same for both sides of the conflict? From the secure viewpoint of the purely descriptive, lawyers might confidently describe some of these questions as settled. But many others, even from the vantage point of descriptive theory, remain under-analyzed and radically lacking in clarity and certainty.