How has the EEAS affected, if in any way, the coherence of the CFSP on the vertical dimension in security and defence matters? In this thesis I answer this research question in order to draw a conclusion on the affect of the EEAS on the cooperation and coordination of the MS with and within the CFSP in security and defence matters. Coherence within the EU’s foreign policy is an issue that its leaders have emphasized on developing for more than four decades. The introduction of the EEAS through the Lisbon Treaty in 2009 is perceived to enhance the cooperation and coordination of the MS with and within the CFSP, as well as coordinate the actions of the MS outside the boundaries of the EU. By conducting a multiple case study, in this thesis I compare how the most powerful MS: the UK, France, and Germany, interacted with and within the CFSP in the 2003 Iraq War, and the 2011 military intervention in Libya. Relatedly, a qualitative content analysis approach is applied in order to uncover all relevant information from the primary and secondary literature concerning the two cases. In contrast to most of the previous studies conducted on this topic, I combine and apply the theoretical perspectives of liberal institutionalism and social constructivism in order to explain and explore the phenomenon under investigation. Conclusively, this thesis suggests that the coherence of the CFSP on the vertical dimension in security and defence matters has not been affected in any way after the EEAS has been implemented in the CFSP. This is due to the inherent characteristic of the MS of always prioritizing their national interests, determined by their value-based normative ideas gained through social interaction, before a collective, effective, and coherent CFSP in defence and security terms.