This research is aimed at examining the interacting aspects of Brexit and the EU’s Common Security and Defence Policy (CSDP) and the effects of the former on the latter in the coming years. Many scholars and experts have presented a view that Brexit would not affect the CSDP in significant ways as the CSDP has been administered essentially by an inter-governmental method and as the level of cooperation and integration among the EU Member States (MS) has remained still very low. A group of other experts, however, insist that Brexit would affect the CSDP greatly than expected when we look at the central role played by the UK along with France ever since the creation of the CSDP in the late 1990s. They understand that the trends of cooperation and competition between these two countries have always shaped much of the course of CSDP development and that the field of security and defence has also been integrated deeply than thought among the MS. In addition, in view of the negative role played by the UK in inhibiting the progress in the CSDP from time to time, the UK’s withdrawal from the EU would highly likely induce far greater changes than most people could foresee. To know exactly the extent to which Brexit would impact upon the CSDP and the ways in which the CSDP would evolve in the coming years is an arduous task, but it is very important to know about it as it would determine the course of European integration as a whole as well as the evolution of the CSDP in the coming years. This paper explores first the changing relationships between the UK and the EU’s CSDP, then investigates the impacts of Brexit on the CSDP. It will be followed by the discussions on the recent proposals for the creation of the European Defence Union (EDU) and other initiatives for European defence integration.