Almost nobody disputes that the end of the Cold War had a profound impact on the whole pattern of international security but, more than a decade after the transition, the character of the post-Cold War security order still remains hotly contested. This book explores the idea that, since decolonisation, the regional level of security has become both more autonomous and more prominent in international politics, and that the ending of the Cold War accelerated this process (Katzenstein 2000). This idea follows naturally from the ending of bipolarity. Without superpower rivalry intruding obsessively into all regions, local powers have more room for manoeuvre. For a decade after the ending of the Cold War, both the remaining superpower and the other great powers (China, EU, Japan, Russia) had less incentive, and displayed less will, to intervene in security affairs outside their own regions. The terrorist attack on the United States in 2001 may well trigger some reassertion of great power interventionism, but this is likely to be for quite narrow and specific purposes, and seems unlikely to recreate the general willingness to intervene abroad that was a feature of Cold War superpower rivalry. The relative autonomy of regional security constitutes a pattern of international security relations radically different from the rigid structure of superpower bipolarity that defined the Cold War. In our view, this pattern is not captured adequately by either ‘unipolar’ or ‘multipolar’ designations of the international system structure.