Since 2005, the EU has been engaged in accession negotiations with Turkey. To this day, there is a controversial discussion about Turkey's suitability for Europe and the Union's capacity to absorb new members. The study contributes to the foreign, security, and defense policy dimension. The EU now plays a global political role in this policy area, but it has a different foreign policy culture than Turkey, which creates conflicts. However, processes of Europeanization will continue to lead to rapprochement between Brussels and Ankara. Based on this thesis, the author analyzes the impact of potential Turkish membership on the EU's decisions, power structures, and capacities. Case studies empirically support the findings: What role does Ankara play in the relationship between the EU and NATO? Improved EU-NATO cooperation, the acceleration of the EU's security autonomy, or even the intensification of transatlantic rivalry are possible developments. Additionally, the opportunities, potentials, and risks of Turkey's EU accession for the crisis regions of the Middle East and South Caucasus are considered.