Cambodian genocide – Wikipedia

In 1968, the Khmer Rouge officially launched a nation-wide insurgency across Cambodia. Even though the government of North Vietnam had not been informed of the Khmer Rouge’s decision, its forces provided shelter and weapons to the Khmer Rouge after the insurgency began. North Vietnamese support for the Khmer Rouge’s insurgency made it impossible for the Cambodian military to effectively …

https://en.wikipedia.org/wiki/Cambodian_genocide