After decades in which NATO epitomized the central forum towards the pursuit of European defense, a progressing integration and enlargement process of the European Union’s defense sector has catalyzed a debate about the future of the European defense and security architecture. The implementation of collective EU defense structures like the Permanent Structured Cooperation in 2017 aggravated concerns about a duplication of needs for military and defense capabilities and consequently a diminishing role of NATO, particularly among non-EU NATO states. Taking this background into account, the study aimed to elaborate the influence of both NATO and EU – as institutions commissioned with defense and security endeavors - on their member states’ military sectors. In particular, how member states aligned their military sectors with institutional expectations towards members’ commitment and how these developments could be explained in course of a historical institutionalist approach, in particular by the concept of path dependency. For this matter, the study revisited developments in member states’ military sectors from 1996 – when the European Defense and Security Identity was agreed on – until today in a first step, connected to an analysis on the extent to which the identified developments could be traced back to the states’ membership in both NATO and EU as driving factors. By using path dependency as explanatory variable, the study ultimately aimed to identify dimensions in which a member’s commitment to EU and NATO constituted a “path” that would shape decision-making towards domestic military sectors - e.g. in form of member states’ compliance with norms and guidelines or engagement in institutions’ operations - for years to come. The cases selected for the study were France, Germany and the United Kingdom which after the Brexit is still committed to EU defense structures in course of 3rd state participation.