Synopsis The idea of collective security is novel in the sense that it differs fundamentally from all other security arrangements made by States historically, though it is not entirely new. Past efforts to regulate international relations never really aspired to anything quite so universal. In contrast, collective security aspires to an unprecedented degree of universality, by requiring every member of the system to act. The apparent common sense and appealing simplicity of the logic of collective security can be contrasted with the difficulties of its application. The United Nations Charter has elaborated both the concept and actual dimensions of collective security. This book is a modest attempt to analyze critically the theory and practice of collective security under the auspices of United Nations.