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.