This chapter draws attention to the network of organizations and organizational interactions that mark the European security landscape. We uncover the politics surrounding (possible) European security cooperation and examine the European Union (EU)'s role in these inter-organizational dynamics. To do so, we begin with a conceptual discussion of organizational overlap as a suitable framework for understanding European security governance. We then explore the origins and characteristics of organizational overlap in Europe, focusing on the politics between the EU and NATO in addressing urgent security challenges, particularly the 2022 full-scale Russian invasion of Ukraine and the second Trump administration. The chapter continues by analysing the implications of organizational overlap for the European security order. As we illustrate, organizational overlap has resulted in compartmentalized multilateralism along functional and geographical lines rather than the orchestration through organizational hierarchies and deference. The analysis concludes with suggestions for further research.