On April 18, 1978, Pol Pot, fearing a Vietnamese attack, ordered a pre-emptive invasion of Vietnam. His Cambodian forces crossed the border and looted nearby villages, mostly in the border town of Ba Chue. Of the 3,157 civilians who had lived in
Ba Chúc,
[65] only two survived the massacre. These Cambodian forces were repelled by the Vietnamese.
[66]
By December 1978, due to several years of border conflict and the flood of
refugees fleeing
Kampuchea, relations between Cambodia and Vietnam collapsed. On December 25, 1978, the
Vietnamese armed forces, along with the
Kampuchean United Front for National Salvation, an organization that included many dissatisfied former Khmer Rouge members,
[67] then invaded Cambodia, capturing
Phnom Penh on January 7, 1979. Despite a traditional Cambodian fear of Vietnamese domination, defecting Khmer Rouge activists assisted the Vietnamese, and, with Vietnam's approval, became the core of the new
People's Republic of Kampuchea, quickly dismissed by the Khmer Rouge and China as a "
puppet government".
[66]
At the same time, the Khmer Rouge retreated west, and it continued to control certain areas near the Thai border for the next decade. These included
Phnom Malai, the mountainous areas near
Pailin in the
Cardamom Mountains, and
Anlong Veng in the
Dângrêk Mountains.
[68]
These Khmer Rouge bases were not self-sufficient and were funded by diamond and timber smuggling, by military assistance from China channeled by means of the Thai military, and by food smuggled from markets across the border in Thailand