The thesis examines the inter-organizational relationship between the European Union (EU) and the North Atlantic Treaty Organization (NATO) as the ‘sleeping princess’ of the Lisbon Treaty – also known as Arts. 42(6) and 46 TEU or Permanent Structured Cooperation (PESCO) – has been awoken. The PESCO articles were invoked in the late 2017 during the Estonian Presidency of the Council of the EU, which entail an ambitious, binding and inclusive European legal framework for investments in security and defence. Furthermore, the PESCO articles are output oriented and should enable tangible progress on collaborative capability development goals, and the availability of deployable defence capabilities for combined missions and operations.

Concerns has been raised regarding the new development of these provisions because of how the PESCO articles may dilute NATO, as the cross-organizational member states could focus their military and defence resources towards the EU and the newly activated provisions. Thus, keeping the EU and NATO as separate organizations, of which they could rely on regarding military defence, thereby riding two horses at once.

As both the EU and NATO could end up attending to military deployment and crisis management, it is important to include both organizations in the aspect of foreign security and defence. Therefore, this thesis will be analyzing the inter-organizational relationship between the EU and NATO through a multilevel framework analysis in order to explore different aspects where the two organizations promote cooperation or aspects more prone to inter-organizational rivalry. This will provide hints as to whether there is something to the fact that the EU is riding two horses, or if it’s a totally different kettle of fish.

In addition hereto, this choice to invoke the PESCO articles will be discussed from a neoclassical realist perspective. Though utilizing a theory of International Relations (IR) might seem like an odd choice, as there is no specific IR theory of inter-organizational relations, neoclassical realism will provide with additional arguments for why the PESCO articles have been invoked.

The findings of the inter-organizational multilevel analysis show that there are a great deal of cooperation between the EU and NATO. However, the findings also show small tensions on member state level both between some cross-organizational member states as well as non cross-organizational member states. Yet, the overall relationship can be regarded as co-operative – at least to the best of their capabilities.

By utilizing the concepts of balancing, the discussion shows that for the cross-organizational member states, it is no longer viable to solemnly relying on NATO. In order for the EU member states to be safeguarded from potential aggressive states, it is therefore also important to actively increase their won military capabilities. This, however, does not mean without NATO. The EU member states must do this in cooperation with NATO and it must be an addition to the military defence partnership established by NATO.