This volume of essays examines the development of political and legal thinking regarding the use of force in international relations. It provides an analysis of the rules on the use of force in the political, normative and factual contexts within which they apply and assesses their content and relevance in the light of new challenges such as terrorism, weapons of mass destruction and cyber-attacks. The volume begins with an overview of the ancient and medieval concepts of war and the use of force and then concentrates on the contemporary legal framework regulating the use of force as moulded by the United Nations Charter and state practice. In this regard it discusses specific issues such as the use of force by way of self-defence, armed reprisals, forcible reactions to terrorism, the use of force in the cyberspace, humanitarian intervention and the responsibility to protect. This collection of previously published classic research articles is of interest to scholars and students of international law and international relations as well as practitioners in international law.