At the end of the Second Indochina War (more popularly known in the United States as the Vietnam War), the Soviet Union, the People’s Republic of China, and Democratic Kampuchea were among Vietnam’s closest allies. At the same time, the new Socialist Republic was hoping to establish diplomatic relations with many countries that had been allies of the Republic of Vietnam (South Vietnam) during the war, including the then five-member Association of Southeast Asian Nations (ASEAN) and the United States itself, hoping to capitalize on new trade and investment opportunities to rebuild its tattered economy and avoid overdependence on a single great power benefactor. Yet as early as 1978, this dream had collapsed as Vietnam found itself in the unenviable position of becoming reliant on Soviet economic support to fight the first full-scale conflict between socialist nations – a two-front war against both China and Cambodia. Vietnamese troops would remain bogged down in a bloody guerrilla war in Cambodia until 1989. Not until 1991 would the parties finally agree to a political solution to the conflict, and Vietnam became the first socialist country to become a full member of ASEAN in 1995. My central argument is that in terms of foreign economic policy, Vietnam consistently sought from 1975 onwards to diversify trade relations and to not become overly dependent on aid from a single power. In the 1970s, Vietnam tried unsuccessfully to avoid the Third Indochina War, which would jeopardize its quest for independence through multilateralism. When it finally did invade Cambodia primarily as an act of self-defense, Vietnamese leaders found withdrawal politically impossible as they committed to justifying the original invasion post facto as a humanitarian intervention. While the Vietnamese domestic economy changed significantly with the doi moi reforms in 1986, Vietnam’s economic integration in the 1990s was therefore not a revolutionary break from a conservative past but rather a fulfillment of a vision in the 1970s, with the notable difference that Vietnam and other ASEAN countries would through the Third Indochina War elevate absolute state sovereignty and non-interference to be the most important principles guiding regional affairs. In situating my work at the intersection between the International Relations debate on the nature and driving force of regionalism and the historical debates surrounding the Cambodian Genocide and the Third Indochina War, I hope my research will attract a wide audience of scholars, practitioners, and the interested public.