Originally published in 1936, this book presents the content of an inaugural lecture delivered by the renowned British legal expert Arnold McNair (1885–1975), upon taking up the position of Whewell Professor of International Law at Cambridge. McNair was also a judge of the International Court of Justice and was the first President of the European Court of Human Rights. The text presents a discussion of the role of collective security and 'the true place of force in a system of international law'. This book will be of value to anyone with an interest in legal history, international relations and the works of McNair.