The EU and NATO are historic partners. Since the end of the last century, both organisations have worked together in the field of defence to ensure peace and security on the European continent. Common values and challenges have made their partnership one of the most successful in the world. However, the EU-NATO relationship has traditionally been legally unclear and not fully operational, with stops and starts. In particular, NATO seemed lost after the disintegration of the Union of Soviet Socialist Republics, and the EU has never proved to have sufficient military capabilities to play a security role on its own.
This article seeks to understand the legal basis of the EU-NATO partnership, with the ultimate aim of demonstrating that it has been effective throughout history. It tries to systematise the legal acts of both organisations in a new and coherent way, revealing the extraordinary nature of this relationship. As this contribution argues, EU-NATO cooperation is not only legally anchored in the meaning of the EU and NATO Treaties. Still, it is also put into practice through mutual participation in their respective fields of action.