Abstract The Third Indochina War called forth dramatic changes in the international relations of Southeast Asia. Foremost among these changes was a shift in the geopolitical orientation of the Association of Southeast Asian Nations (asean). The organization’s founders established asean in 1967 to contain Chinese influence in Southeast Asia. But in the wake of the Vietnamese invasion of Cambodia in late 1978, asean forged a tacit alliance with the People’s Republic of China to pursue a campaign of rollback against Vietnam. This article argues that asean’s volte-face and China’s reentry into networks of regional diplomacy depended upon a shift in Indonesian threat perceptions. As political Islam displaced Chinese communism as the key threat fixating Indonesian policy, the Suharto regime abandoned its longstanding attempts to integrate Vietnam into the architecture of regional order and instead accommodated itself to a Thai-led effort to enlist China as a counterweight against Vietnam. The reorientation of Indonesian diplomacy reveals the dynamics of a phenomenon that anthropologist Heonik Kwon has called the “decomposition” of the Cold War – the geographically and temporally uneven erosion of the Cold War as a social reality and the gradual elaboration of a post-Cold War era.