This article advances a practice‐approach to differentiated (dis)integration in European security and defence in light of Brexit. We propose that understanding how differentiation in the area of security and defence is given meaning on the ground requires examining the everyday social practices of officials and military professionals. Therefore, we focus on the “low politics” of security and defence cooperation in Europe. We do so by entering the diplomatic “engine room” in the EU's Political and Security Committee. We show how arguments that Brexit will be the ultimate death blow to the EU's Common Security and Defence Policy or that it will be the catalyst for deeper military cooperation, fail to take account of the social dynamics and the historical path of existing security and military cooperation in Europe. Future research on differentiation should pay particular attention to how such mechanisms plays out in everyday social negotiation of meaning among practitioners.