Struck by decades of insufficient funding and lack of political will, European defence has not grown in proportion to other sectors of European involvement. However, major recent crises, such as the Syrian war, the unprecedented migratory and refugee crisis and the wave of terrorist attacks across the European continent have forced EU actors to realize that European security is inextricably linked with an assumption of the EU’s political role on the world stage. The publication of the EU Global Strategy in 2016 and the Reflection paper on the future of European defence in the following year, paved the way for the launching of the Permanent Structured Cooperation in December 2017. Yet, the UK’s participation in European defence in the aftermath of Brexit and EU relations with NATO will be the litmus test of PESCO.