This paper focuses on the European Security and Defense Policy and tries to answer the question why the member states have deviating deepening preferences in this field. After discovering the supranational-intergovernmental divide, we will determine its possible causes. The Qualitative Comparative Analysis helps us to distillate the most valuable independent variables and reveals ‘institutional culture’ and ‘balancing’ as most important. Finally, we match our findings with the initial theories, used to derive the causal variables, and verify their explanatory power.