The security landscape in Europe since the end of the Cold War has been marked by the dominance of NATO as the sole credible structure for military security. This situation is undergoing a transformation. There is now the existence of an embryonic division of labour between the European Union, through its ESDP, and NATO. To understand how this situation has arisen requires an understanding of the manner in which the EU has developed a military security identity and this article will commence by examining the recent historical development of the ESDP. The article then proceeds to identify the nature of the emerging division of labour between the EU/ESDP and NATO. The examination is structured by identifying four sets of reasons behind the development of the emergent division of labour: the development of an EU consensus on military security; the 'new' NATO dynamic; atrophy in transatlantic relations; and finally, operational developments.