This book is about the evolution of International Security Studies (ISS), in the beginning as an independent field of study, but quite quickly absorbed as a sub-field of International Relations (IR), which was developing rapidly alongside it. Like IR itself, ISS is mainly a Western subject, largely done in North America, Europe and Australia with all of the Western-centrisms that this entails. ISS is one of the main sub-fields of Western IR. Wherever IR is taught, ISS is one of its central elements. There is an antecedent literature extending back before the Second World War which can largely be characterised as war studies, military and grand strategy, and geopolitics. This includes much discussed writers such as Clausewitz, Mahan, Richardson and Haushofer, whose work still remains relevant. But we are not going to cover this literature both for reasons of space, and also because a distinctive literature about security developed after 1945 (Freedman, 1981a; Wæver and Buzan, 2007). This literature was distinctive in three ways. First, it took security rather than defence or war as its key concept, a conceptual shift which opened up the study of a broader set of political issues, including the importance of societal cohesion and the relationship between military and non-military threats and vulnerabilities. The ability of security to capture the conceptual centre of ISS dealing with defence, war and conflict as well as the broadness of the term was famously condensed in Wolfers's definition of security as an ambiguous symbol.