Growing apart together? Brexit and the dynamics of differentiated disintegration in security and defence

Brexit initially raised the prospect of new forms of external differentiation in the European Union (EU), should the United Kingdom continue to participate in a number of the Union’s policy areas. Security and defence was one area where agreement on the terms of UK participation was more likely, given the clear interests of both sides in the development of a close partnership in this area. But agreement has been so difficult to reach, and the final Brexit deal makes no mention of collaboration in foreign, security and defence policy. We argue that the key to understanding this puzzle lies in understanding the politics of differentiated disintegration, of which Brexit is the prime example, and the distinction between strategic and political interests. While strategic interests constitute a driver for external differentiation, the political interests arising from the withdrawal process make it difficult to reach an agreement. Divorcing strategic cooperation from the short-term politics of negotiations is the first step to overcoming the stalemate, and this chapter presents several ways this can be achieved. By perceiving Brexit as a case of differentiated disintegration, this chapter accounts for the significant constraints associated with external differentiation as a mode of integration in the EU.