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