Just war theory exists to stop armies and countries from using armed force without good cause. But how do we decide whether a use of armed force is just or unjust? In this original book, John W. Lango takes some distinctive approaches to the ethics of armed conflict. 1. A revisionist approach that involves generalising traditional just war principles, so that responsible agents can apply them to all forms of armed conflict. 2. A cosmopolitan approach that features the Security Council. 3. A preventive approach that emphasises alternatives to armed force, including negotiation, nonviolent action and peacekeeping missions. 4. A human rights approach that encompasses not only armed humanitarian intervention but also armed invasion, armed revolution and all other forms of armed conflict. Using these principles, he discusses issues surrounding just cause, last resort, proportionality and noncombatant immunity. He then applies them to hot topics in international conflicts including drone strikes, no-fly zones, moral dilemmas, deterrence, intelligence, legitimate authority, escalation and peace agreements, drawing on real-world case studies from recent conflicts in countries including Afghanistan, Darfur, Libya and South Sudan.