This book analyzes the concept of military necessity and just war thinking and argues that it should be seen as a vital moral principle for leaders. The principle of military necessity is well understood in the manuals of modern militaries and is recognized in the war convention. It is the idea that battlefield commanders should make every effort to win on a local battlefield, within legal means, and using proportionate and discriminating weapons and tactics. Every legal textbook on war includes military necessity as a foundational principle within the jus in bello (ethics of fighting war) alongside principles of proportionality and distinction, and it is taught in every Western military academy. Even the International Committee of the Red Cross lauds the concept as a cardinal principle of warfare. However, unlike legal scholarship, one can pick up a book by almost any just war thinker in philosophy, theology, or the social sciences, and the concept is missing altogether in their literature. This volume returns military necessity to just war thinking and lays out the argument for doing so. Each contributor taps into one of the many dimensions of military necessity, such as its relationship to jus ad bellum (ethics of going to war) categories (e.g., right intention), its relationship to jus in bello categories, or its application in foreign policy and military doctrine. Case studies in the book point out the practical moral dimensions of military necessity in cases from the targeted killing of terrorists to battlefield decisions that led to the use of the atomic bomb at Hiroshima. This book will be of interest to students of just war theory, military ethics, statecraft, and international relations.