The United Kingdom (UK) Prime Minister Theresa May’s official request, on March 29th, to leave the European Union (EU) will change the way European countries organise their defence policies. Europeans have always managed their defence via various channels: nationally, in the EU, through NATO and in smaller formats. Brexit will change the way these formats function and how they interact. The effect on the EU’s Common Security and Defence Policy (CSDP) will be limited. NATO might benefit from a greater UK commitment, as can smaller formats. The result will be greater fragmentation in European defence, which risks weakening the Europeans’ political and military capacity to act.