Security and Defence is a complex area within the EU. In spite of a long-standing history, its development and role within the EU has long remained distinct from other areas; the particular attachment of this area to state sovereignty has necessitated a cautious approach. This paper explores the recent developments in Security and Defence since the Brexit vote in 2016, arguing that, rather than the prospect of a UK withdrawal causing a weakening in the EU in this field, instead, it has seen a reinvigoration. It is argued that this is reflective of a broader development of the EU in terms of its autonomy as an external actor, which has shaped developments and continues to do so. While this is beneficial in some ways, the paper concludes by exploring some of the potential problems that may arise from the EU’s particular approach to autonomy as the Brexit process continues.