Differentiated Cooperation in the EU’s Foreign and Security Policy: Effectiveness, Accountability, Legitimacy

European Union (EU) treaties have introduced legal frameworks for differentiated integration in European foreign and security policy, but they have rarely been used. Instead, member states have engaged in informal practices of differentiated cooperation. Based on an analysis of effectiveness, accountability and legitimacy of differentiated cooperation in the Western Balkans, the Middle East Peace Process, negotiations on Iran’s nuclear programme and the Ukraine crisis, we argue that differentiated cooperation has had positive outcomes when it has adhered to common EU values and positions. When this has not been the case, differentiation has undermined EU foreign and security policy.