How can the regulations of the single market have entered into the Common Security and Defense Policy (CSDP) of the European Union (EU) when they were originally intended to have been excluded from it? In order to understand this transfer to the CSDP of regulations originating with the single market in the “field of Eurocracy”, this chapter uses the concept of “communities of practice”. It applies this concept to the case of the “defense package”, which corresponds to the first two directives that govern arms policy within the EU. The adoption of the “defense package” by the Council and Parliament in 2009 can be explained by the rivalry between communities of “supranational” practice and of “intergovernmental” practice. The national and European actors who make up the community of supranational practice were in favor of the “defense package” because they wished to expand the CSDP’s legitimate institutional perimeter. On the other hand, the community of intergovernmental practice consisted of actors who opposed the “defense package” in order to reproduce the CSDP’s existing institutional boundaries. This analysis of the adoption of the “defense package” summarizes the structuring of the field of Eurocracy. The operationalization of this endogenous argument has required the production of primary data from a field survey (28 semi-structured interviews). This chapter draws upon a sociological approach to contribute to the literature on the circulation of regulations at the international level. Taking as its starting point the transfer of single market regulations to the CSDP, it also sheds light on the work devoted to the circulation of EU regulations to member states in the areas of defense and foreign policy as well as the circulation of regulations outside the EU.