The United States has been the most powerful country in the world since the Second World War. In the wake of September 11, at a time when the US has an unrivalled ability to project power in any part of the globe, this may seem like a remarkably anodyne observation. Yet it is important to remember that for much of the postwar period the Soviet Union was a formidable adversary that constrained American influence and provided an alternative vision of the way the world might be ordered. We now know, of course, that the Soviet system was incapable of supporting either its military pretensions or the aspirations of much of its citizenry, but this should not blind us to the fact that for many years ideological rivalry and superpower confrontation were the seemingly immutable realities of the postwar order. In the wake of the Soviet Union's disintegration, and despite periodic concerns about the performance of the US's own economy, America has emerged as the sole-superpower and the cornerstone of what is routinely depicted as a unipolar interstate system (Wohlforth 1999). Over the last 50 years or so, therefore, American power has waxed, waned and waxed again. This is interesting enough in itself, but from the perspective of Southeast Asia it is especially important because the changing nature of America's global ambitions and its capacity to achieve them has coincided with a critical phase of nation-building and economic development across Southeast Asia. The intention of this chapter is to examine the impact that the US has had on Southeast Asia's historical development, both during the Cold War period when the emergent states of the region attempted to consolidate and assert their independence, and more recently, when the combined effects of economic fragility and the emergence of new strategic challenges have provided a painful reminder of just how susceptible the region remains to powerful external forces over which it has limited influence (Beeson 2002).