This book develops a set of cosmopolitan just war principles for all forms of armed conflict. Just war theory exists to stop armies and countries from using armed force without good cause. But how can we judge whether a war is just? John Lango adopts a cosmopolitan approach to argue that the more traditional state-centric just war theory should be both globalised and democratised. From this foundation, he formulates a set of morally absolute principles. He shows how these can be applied to all forms of armed conflict, however large or small: from interstate wars to UN peacekeeping missions, and from civil wars to counter-insurgency and counter-terrorism operations.