This book examines the evolution of the EU's Common Security and Defence Policy (CSDP) from its inception in 1998 to the present day. Using the theoretical framework of historical institutionalism, it examines both the successes and failures of the CSDP. Drawing on a series of interviews with officials and researchers from various EU institutions, NATO, and diplomatic missions of EU member states, it assesses what has instigated changes in the CSDP, and why some events have proven more determining and influential than others. The book reviews six crises that have shaped the CSDP, including the Yugoslav Wars, the Second Gulf War, the Libyan campaign, the Ukrainian crisis, the Syrian crisis, and Brexit, in order to understand how real-life events have influenced policy. In this context, the book defines the term 'European Strategic Autonomy' dynamically, as the residual effect of negotiation over time. It will appeal to government officials and policymakers, as well as students and scholars of European politics and international relations.