The War on Terror has remained an enduring feature of American foreign policy for over two decades. This short history positions the War on Terror within the broader context of Cold War interventionism and the rise of various transnational threats to American (and global) security during the late twentieth century. It introduces readers to the main concepts, debates and theories which have been used to understand and make sense of the War on Terror. These include approaches that frame it as a disparate set of policies aimed at reducing the risk of terrorist attacks against American citizens at both home and aboard; as part of a project aimed at helping maintain the United States' dominant position within international politics; and as an idea intimately bound up with perceptions of American national identity and America's place in the world. In this way, the book aims to show how the War on Terror has changed global politics, as well as why it has been fought and proven so difficult to end despite multiple failed attempts at course correction. The book is ideally suited for courses on international security, American foreign policy and contemporary world politics.