The NATO-ESDP relationship is studied in light of the wider transatlantic relations. Over the last two decades America and Europe have come through a continuously changing relationship on matters of security and defence. First the collapse of the Soviet threat in Europe, in early 1990s, meant that America 's security interests outside Europe now became more important. While the now more integrated European Union was struggling to overcome its member-national differences and codify a security and defence agenda. The EU developed the European Security and Defence Policy in 1999, its military agency.

The transatlantic security relationship develops into a NATO-ESDP relationship ever since. In 2001 the rise of new terrorism brings America and Europe closer than ever before since 1990. The new security and defence is also a global one, to face which both sides of the Atlantic must cooperate. Yet this cooperation has not been as blissful as desired. America and the EU disagreed heavily upon the war on Iraq in 2003. Nevertheless NATO and ESDP have agreed a couple of significant cooperation treaties between them, concerning primarily the further development of ESDP capabilities, and this has been a vital step so far. Finally this study aims at drawing elaborate conclusions about the development NATO and ESDP cooperation so far in order to understand how it should proceed in future.