The People’s Republic of China (PRC) has loomed large in the geopolitical calculations of Southeast Asian countries since 1949. Geography and demographics provide an obvious explanation: China is more than double the size of Southeast Asia in terms of both landmass and population; it is contiguous with the region, sharing long land frontiers with Vietnam and Myanmar, and a short one with Laos; and due to its expansive maritime claims, Beijing has overlapping sovereignty claims over a profusion of small reefs and atolls in the South China Sea with Vietnam, Malaysia, the Philippines and Brunei, and contested sea boundaries with those four countries plus Indonesia. Other salient factors have included the sensitive relationship between the mainland Chinese and their ethnic brethren resident in Southeast Asia; Beijing’s ideologically driven foreign policy during the Cold War; and, since the early 1990s, China’s phenomenal economic progress and commensurate growth in military power. Most importantly, perhaps, it has been in Southeast Asia that China’s ideological and strategic rivalry with the other Great Powers has been played out, often forcing regional states to choose sides. Competition between the PRC and other Great Powers for influence in Southeast Asia has continued into the twenty-first century, and has the potential to increase if US-China relations worsen.