Pol Pot

=== Vietnamese Invasion: 1978–1979 ===
{{Main|Cambodian-Vietnamese War}}

On 25 December 1978, the Vietnamese Army launched its full-scale invasion.{{sfn|Short|2004|p=395}} Its columns initially advanced into north-east Cambodia, taking [[Kratié (town)|Kratie]] on 30 December and [[Stung Treng]] on 3 January.{{sfn|Short|2004|p=395}} The Vietnamese main force then entered Cambodia on 1 January 1979, heading along Highways one and seven toward Phnom Penh.{{sfn|Short|2004|p=395}} Cambodia's forward defences failed to stop them.{{sfn|Short|2004|p=397}} With an attack on Phnom Penh imminent, in January Pol Pot ordered Sihanouk and his family to be sent to Thailand.{{sfn|Short|2004|pp=396–97}} The entire diplomatic corps followed shortly after.{{sfn|Short|2004|p=396}} On 7 January, Pol Pot and other senior government figures left the city and drove to [[Pursat]].{{sfn|Short|2004|p=398}} They spent two days there before moving on to [[Battambang]].{{sfn|Short|2004|p=402}}

After the Khmer Rouge evacuated Phnom Penh, Mok was the only senior government figure left in the city, tasked with overseeing its defence.{{sfn|Short|2004|p=398}} Nuon Chear ordered the cadres in control of S-21 to kill all remaining inmates prior to it being captured by the Vietnamese.{{sfn|Short|2004|p=400}} However, the troops guarding the city were unaware how close the Vietnamese Army actually were;{{sfn|Short|2004|p=400}} the government had concealed the extent of the Vietnamese gains from the population.{{sfn|Short|2004|p=399}} As the Vietnamese approached, many officers and other soldiers guarding the city fled; the defence was highly disorganised.{{sfn|Short|2004|pp=400–01}} There were isolated examples of Cambodian villagers killing Khmer Rouge officials in revenge.{{sfnm|1a1=Chandler|1y=1992|1p=165|2a1=Short|2y=2004|2p=401}} In January, Vietnam installed a new government under Samrin, composed of Khmer Rouge who had fled to Vietnam to avoid the purges.{{sfn|Short|2004|p=409}} The new government renamed Cambodia the "[[People's Republic of Kampuchea]]".{{sfnm|1a1=Chandler|1y=1992|1p=165|2a1=Short|2y=2004|2p=409}} Although many Cambodians had initially hailed the Vietnamese as saviours, over time resentment against the occupying force grew.{{sfn|Short|2004|p=409}}

The Khmer Rouge turned to China for support against the invasion. Sary travelled to China via Thailand.{{sfn|Short|2004|p=402}} There, Deng urged the Khmer Rouge to continue a guerrilla war against the Vietnamese and to establish a broad, non-communist front against the invaders, with a prominent role given to Sihanouk.{{sfn|Short|2004|pp=402–03}}
China sent its vice premier, [[Geng Biao]], to Thailand to negotiate the shipment of arms to the Khmer Rouge through Thailand.{{sfn|Short|2004|p=405}} China also sent diplomats to stay with the Khmer Rouge encampments near the Thai border. Pol Pot met with these diplomats twice before the Chinese government withdrew them for their safety in March.{{sfn|Short|2004|pp=406–08}} In China, the Khmer Rouge set up their "Voice of Democratic Kampuchea" radio station, which remained their main outlet for communicating with the world.{{sfn|Short|2004|p=402}} In February, the [[Sino-Vietnamese War|Chinese attacked northern Vietnam]], hoping to draw Vietnamese troops away from the invasion of Cambodia.{{sfn|Short|2004|p=407}} As well as China, the Khmer Rouge also received the support of the United States and most other non-Marxist southeast Asian countries who feared Vietnamese aggression as a tool of Soviet influence in the region.{{sfn|Short|2004|p=406}}

On 15 January, the Vietnamese reached [[Sisophon]].{{sfn|Short|2004|p=405}} Pol Pot, Nuon Chea, and Khieu Samphan then moved to [[Pailin Province|Palin]] on the Thai side of the border, and in late January relocated again, to [[Tasanh]], where Sary joined them. There, on 1 February, they held a Central Committee conference, deciding against Deng's advice about a united front.{{sfn|Short|2004|p=406}} In the second half of March, the Vietnamese moved to hem in the Khmer Rouge along the Thai border, where many of Pol Pot's troops had crossed into Thailand itself.{{sfn|Short|2004|p=408}} The Vietnamese advanced on Tasanh, from which the Khmer Rouge leaders had fled only a few hours before it was captured.{{sfn|Short|2004|pp=407–08}}