State recourse to the use of force has long been considered the main threat to international peace and security. The rules governing resort to force form a central element within international law. This book, therefore, is devoted to deal with jus ad bellum. First, the regulation of war in the early days of international law until 1945 is discussed. Then, it focuses on the legality of the use of force in the post-Charter era. It begins by discussing the prohibition of the use of force under the UN Charter. After that, it analyses the justifications for the use of force in terms of self-defence and authorised force by the Security Council. It also argues whether the customary right of anticipatory self-defence was abolished by the Charter and whether such a right has been revived in the period after 1945. Next, this book explores many issues that are the subject of ongoing and longstanding debate such as the Legitimacy of humanitarian intervention and the "protection of nationals"doctrine. Finally, it determines the consequences of an illegal use of force. This book will be of great interest to researchers and students of international law, conflict and Security Studies in general.