In what circumstances is it legitimate to use force? How should force be used? These are two of the most crucial questions confronting world politics today. The Just War tradition provides a set of criteria which political leaders and soldiers use to defend and rationalize war. This book explores the evolution of thinking about just wars and examines its role in shaping contemporary judgments about the use of force, from grand strategic issues of whether states have a right to pre-emptive self-defence, to the minutiae of targeting and interrogation tactics. Bellamy maps the evolution of the Just War tradition, demonstrating how it arose from a myriad of sub-traditions including scholasticism, the holy war tradition, chivalry, natural law, positive law, Erasmus and Kant's reformism, and realism from Machiavelli to Morgenthau. He then applies this tradition to a range of contemporary normative dilemmas related to terrorism, pre-emption, torture, aerial bombardment and humanitarian intervention.