How relevant is the concept of war today? This book examines how notions about war continue to influence how we conceive legal rights and obligations. It considers situations that recognize the significance of a Declaration of War or a State of War, both domestically and internationally. It outlines how the institution of War was abolished in the 20th century and replaced with a ban on the use of force. At the same time, international criminal law was developed to prosecute wars of aggression and war crimes. The book highlights how states nevertheless continue to claim that they can resort to the use of force, engage in lawful killings in war, imprison law of war detainees and attack objects that are said to be part of a war-sustaining economy. The book provides an overall account of the laws of war and a detailed inquiry into whether states should be able to continue to claim Belligerent Rights over the enemy and neutrals, including those rights connected to booty and blockade. The book claims that while there is general agreement that War has been abolished as a legal institution for settling disputes, the time has come to admit that the Belligerent Rights that states claim flow from being at war are no longer available. Therefore, claiming to be in a war or an armed conflict does not grant anyone a licence to kill people, destroy things, and acquire other people’s property or territory.