There is a paradox in transatlantic security cooperation. In the field of internal security there is evidence of increasing multilateral cooperation, whilst in external security the reverse is the case. The US has moved away from patterns of multilateral cooperation that characterised the Cold War era and has grown to favour bilateral cooperation with particular European allies. This is a reflection of two factors. First, because of its power the US has always possessed a choice over with whom to work: international organisations or individual European governments. The limitations of the organisations, and the problems of obtaining consensus from diverse clusters of states, have always been powerful arguments against multilateralism. Conversely, the inherent strength of the US, the ease of working within bilateral relationships and the opportunity to build coalitions of like-minded states, has been a powerful attraction. Second, US attitudes have changed substantially since 9/11, to a greater extent than those of Europe. Where the US was reluctant to intervene in international crises during the 1990s because it did not perceive its vital interests to be at stake – such as in Bosnia – now the US regards the War against Terror as central to its interests. As a consequence, it has been wary of allowing its power and global reach to be constrained by the attitudes of its European allies, particularly when they add only marginally to US capabilities.