This book examines a long-standing dispute regarding the prerequisite for the exercise of the right to self-defence and aims to offer a possible better alternatives for interpreting the significance of the precondition provided for in the Article 51 of the United Nations Charter, by taking a historical perspective on the development of that concept from the mid-19th century to 1945. The book defines the right of self-defence as understood in and before 1945, suggesting the typology which represents the strata of the concept. It will contribute to the current debate regarding the right of self-defence in contemporary international law, including that against terrorism, by providing a framework to analyse the state practice since 1945.