This book is an examination of the permissions, prohibitions and obligations found in Just War theory, and the moral grounds for laws concerning war. Pronouncing an action or course of actions to be prohibited, permitted or obligatory by Just War theory does not thereby establish the moral grounds of that prohibition, permission or obligation; nor does such a pronouncement have sufficient persuasive force to govern actions in the public arena. So, this work asks, what are the moral grounds of laws concerning war, and what ought these laws to be? Adopting the distinction between jus ad bellum and jus in bello, the author argues that rules governing conduct in war can be morally grounded in a form of rule-consequentialism of negative duties; rules aiming to achieve optimal results regarding the prevention of the violation of certain rights. Rules governing the war decision itself are grounded in self-defence and a duty to rescue; the locus of rightful authority to use force is dependent on institutional design. Looking towards the public rules, the book argues for a new interpretation of some existing law, and in some cases new law. These include recognising rights of encompassing groups to necessary self-defence; recognising a duty to rescue; weapons and tactics that asymmetrically remove risk should be assessed on whether their use violates discrimination; considering all persons neither in uniform nor bearing arms as civilians and therefore fully immune from attack, thus ruling out 'targeted' or 'named' killings; considering all persons who bear arms to have combatant rights (POW status, for example). This book will be of much interest to students of just war theory, ethics of war, international law, peace and conflict studies, and Security Studies/IR in general.