Some member-states of the European Union (EU) want a supranational foreign and security policy, while other member-states oppose any significant limitation of national sovereignty in this domain. What explains this variation? Answering this question could help us to better understand not only the trajectory of European unification, but also the conditions and prospects of consensual political integration in other regional contexts and territorial scales. The main research traditions in international relations theory suggest different explanations. I examine the roles of relative power capabilities, foreign policy interests, Europeanized identities, and domestic multilevel governance in determining the preferences of the fifteen EU member governments concerning the institutional depth of their foreign and security policy cooperation. I find that power capabilities and collective identities have a significant influence, but the effect of ideas about the nature and locus of sovereignty, as reflected in the domestic constitution of each country, is particularly remarkable
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