Most people believe that it is sometimes morally permissible for a person to use force to defend herself or others against harm. In Defensive Killing, Helen Frowe offers a detailed exploration of when and why the use of such force is permissible. She begins by considering the use of force between individuals, investigating both the circumstances under which an attacker forfeits her right not to be harmed, and the distinct question of when it is all-things-considered permissible to use force against an attacker. Frowe then extends this enquiry to war, defending the view that we should judge the ethics of killing in war by the moral rules that govern killing between individuals. She argues that this requires us to significantly revise our understanding of the moral status of non-combatants in war. Non-combatants who intentionally contribute to an unjust war forfeit their rights not to be harmed, such that they are morally liable to attack by combatants fighting a just war.