The Communist Party of Vietnam, and the Khmer Rouge, were nominal allies during the Vietnam War, however the former was allied with the USSR while the latter was allied with the PRC.

Even during the Vietnam War, there was quite a bit of competition between the USSR and PRC for influence in the Viet Minh. Following the Gulf of Tonkin incident in 1964, Deng Xiaoping as secretary general of the CCP offered a large sum to the Viet Minh if they refused USSR aid.

Although the North Vietnamese and Viet Minh agreed to postpone arguing their border disputes with the PRC while the Vietnam War was ongoing, this issue came to fore in the mid-1970 to late-1970s, especially after Nixon visited the PRC. The Sino-Soviet split became even more marked, therefore their respective allies had less reason to not fight each other.

When the Khmer Rouge came to power, they purged Vietnam-trained personnel from their ranks. They feared that Vietnam had designs for a Greater Indochina Communist state. Small scale engagements eventually led to Vietnam's intervention in Kampuchea.