Combined differentiation in European defense: Tailoring Permanent Structured Cooperation (PESCO) to strategic and political complexity

Sustaining meaningful defense cooperation in Europe is made difficult by defense-industrial fragmentation, a multiplicity of institutional frameworks, divergent strategic cultures and domestic opposition to integration. The European Union’s recent foray into defense integration incorporates multiple forms of differentiation to overcome these barriers, with Permanent Structured Cooperation (PESCO) characterized by selective membership, external participation, and project-based clustering. Such “combined differentiation” offers an instructive example of how EU practices and principles can contribute to meaningful defense collaboration, even though Brussels is often thought a weak actor externally. It also illustrates how distinct forms of differentiation can be embodied within a single structure to accommodate complexity in strategic preferences. Using the example of PESCO, this article shows how “combined differentiation” has emerged as a response to the nature of the European defense landscape and how debates between member states about how to respond to specific challenges have brought about further differentiation over time.