Over the last decade, a confluence of strategic and security concerns has threatened the European Union’s survival both within and beyond its political dimension. As a result, security and defence have risen to the top of the EU’s political agenda, culminating in the approval of the EU Global Strategy (EUGS) in 2016. The EUGS represented a watershed moment in the EU’s Common Security and Defence Policy: the EU agreed on ambitious levels of security and defence. The new policy is based on supporting capacity building among member states through instruments such as PESCO. Nonetheless, these instruments have caused variations in patterns of member state behaviour that have enhanced defense integration. This research aims to understand what was the PESCO institutional effect on Member States' preferences and how it has affected the European security and defense goals. The research highlights the role of European agencies and how they contributed to solve collective action problem through a ‘forum effect' on participants, using pro-actively the task of assessing co-operative projects proposals. As a result, PESCO’s institutional effect led to cooperative outcomes between nations that allowed them to overcome coordination dilemmas, namely uncertainty about the willingness to contribute to a common project, which is typical of defense cooperation. Here, we used Rational Choice Institutionalism theory to investigate the PESCO project structure and its interaction with the European Defence policy. Cooperation between participating member states is presented within a cooperative game action, as part of a theoretical approach to game theory. It explains formally how PESCO entails elements to overcome collective action problem among participating member states, while emphasising the institutional design that promoted the European interests, and how this has led to more Europeanised security and defence. Findings are interpreted under the Differentiated integration concept.