External drivers of EU differentiated cooperation: How change in the nuclear nonproliferation regime affects member states alignment

Since its establishment, the Common Foreign and Security Policy (CFSP) has strived to increase convergence among EU member states. Yet, convergence remains elusive and scholars have started to explain the emergence of differentiated cooperation resulting from multiple internal EU crises. We posit that the convergence in the EU member states with respect to nuclear weapons has been fundamentally altered by the humanitarian turn to nuclear disarmament. This has led to a crystallization of differentiated subgroups among the member states, whose membership coincides with that of informal groupings active in the broader nuclear nonproliferation regime. Combining quantitative data on resolution sponsorship at the Non-Proliferation Treaty review process and voting at the UN General Assembly, we show that significant change in the international nuclear nonproliferation regime led to differentiated cooperation within the CFSP, resulting in two cohesive subgroups of member states.