Security and defense issues have been at the top of the European Union’s agenda since British voters opted in favor of Brexit in June 2016. This trend started with the launch of a new strategic document, the Global Strategy for the European Union’s Foreign and Security Policy, which emphasized the need for deepened cooperation and coordination. This chapter provides a detailed process tracing of the major initiatives undertaken after the Brexit referendum in the realm of the Common Security and Defense Policy (CSDP), demonstrating the rising relevance ascribed to the security and defense dimension and the revitalization of the idea of a European Union of Security and Defense. It also scrutinizes the major challenges underpinning the eventual deepening of the CSDP within the post-Brexit EU. The chapter concludes that the European integration process has entered a new phase characterized by a security and defense turn, or the mainstreaming of security and defense, as part of a resilience-building strategy to cope with the uncertainties arising from the prospective withdrawal of the UK from the EU and the Trump Administration’s erratic foreign policy.