For any head of state, foreign policy poses a major challenge as it involves a complex, two-level game famously depicted by Robert Putnam (1988). The more so for a Union of twenty-seven states aiming at coherent policies while having to consider the weight of internal political processes in each member. And yet, what might seem at first an insurmountable challenge has produced several successful, if complex, foreign policies. How can we explain this? This book looks at two of the most important and fruitful foreign policy areas advanced by the European Union: enlargement and security and defence policies (ESDP/CSDP). It aims to explain how domestic political processes allow or limit cooperation between twenty-seven Member States in two policy areas and perhaps in others as well. This is particularly important as the EU enters a new, deeper phase of foreign policy cooperation under the Lisbon Treaty, and as global economic woes have laid bare the difficulties of developing common policies amongst Member States. Understanding the sources that facilitate cooperation and burden-sharing amongst Member States and improved collaboration with partners such as NATO seems particularly pertinent in a rapidly changing and interdependent environment, as sudden events such as the ‘Arab Spring’ and the rise of new powers demonstrate. It is also essential for the continuation of successful policies such as enlargement. For these reasons, this book opens the black box of domestic preference formation. In doing so, it follows and expands Andrew Moravcsik's liberal intergovernmentalism while relying on both quantitative and qualitative methods to uncover state-society relations.