This book operates on two levels. On the more practical level, its overarching concern is to answer the question, When is it permissible to use lethal force to defend people against threats? The deeper concern of the book, however, is to lay out and defend a new account of rights, the mechanics of claims. This framework constructs rights from the premise that rights provide a normative space in which people can pursue their own ends while treating each other as free and equal fellow-agents whose welfare morally matters. According to the mechanics of claims, rights result from first weighing competing patient-claims on an agent, then determining if the agent has a strong enough agent-claim to act contrary to the balance of patient-claims on her, and then looking to see if special claims limit her freedom. The strength of claims in this framework reflects not just the interest in play but the nature of the claims. Threats who have no right to threaten have weaker claims not to be harmed than bystanders who might be harmed as a side effect, all else equal. With this model, a central problem in just war theory can be pushed to the margins: determining when people have forfeited their rights and are liable to harm. Threats may lack a right not to be killed even if they have done nothing to forfeit it.