This is the first comprehensive study of the role, powers and functions of international institutions in the area of peace and security, including both inter-state wars and crises and intra-state situations such as civil wars and serious violations of the rights of individuals and peoples. It examines collective security as one single system consisting of the United Nations and regional security institutions, the foundation of which is laid in Chapter VIII of the UN Charter. The operation of this single system involves multiple ways of interaction between institutions, ranging from collaboration to confrontation. This study draws on the principles that determine the competence of collective security institutions and provide both the guidance for inter-institutional interaction and the criteria of legitimacy of decisions by the relevant institution. The treatment of this area, and of collective security as a whole, is premised on the consensual imperative that allows extending institutional powers only sofar as states have delegated these powers to institutions. This impacts not only on which basis institutions can take action, but also the legal consequences of that action, including the issues of responsibility, judicial review, and implementation of institutional decisions by states--Publisher description.