Chapter 5 analyzes the evolving security structures in East Asia since the end of World War II. What counts as security for the countries in the region and beyond, and the policy choices made accordingly, have made East Asian security the way it is today. Evolution shapes every component of international security, specifically the nation, the nature of politics, and epistemology. Conventional security theories such as the security dilemma and alliance apply to East Asia partly because Western practice and theory have become parts of East Asian practice and theoretical thinking. At the same time, East Asia had a much longer history, and was not a blank canvas for outside influence. The mixture of the old and new explains why East Asian security concepts and practices seem partly familiar and partly strange, which is characteristic of East Asian international relations.