The Vietnamese invasion of Cambodia in December 1978, the overthrow of the Khmer Rouge and the establishment of the People's Republic of Kampuchea, the Sino-Vietnamese war of February to March 1979, and the end of the conflict in Indochina

What this dot point is asking

NESA expects you to explain how the Indochina conflict ended in 1978 to 1979 with the Vietnamese invasion of Cambodia and the Sino-Vietnamese war. Strong answers cover the Khmer Rouge provocations, the Vietnamese diplomatic alignment with Moscow, the invasion and fall of Phnom Penh, the establishment of the People's Republic of Kampuchea, the Chinese punitive invasion, and the longer Cambodian aftermath through to the Paris Peace Agreements of 1991.

The answer

Khmer Rouge provocations

Democratic Kampuchea, from 1977 onwards, claimed the Mekong Delta as Cambodian territory and launched a series of cross-border raids into Vietnam. Khmer Rouge units massacred Vietnamese civilians in An Giang, Tay Ninh, and Kien Giang Provinces. The Ba Chuc massacre (18 April 1978) killed around 3,157 Vietnamese civilians (only two survivors). Ethnic Vietnamese inside Cambodia were expelled or killed; around 150,000 fled in 1977 to 1978.

Pol Pot's regime saw Vietnam as the historic Khmer enemy. The May 1978 Eastern Zone purge, orchestrated by the regime against alleged "Vietnamese minds in Cambodian bodies", killed perhaps 100,000 cadres and drove the survivors (including Heng Samrin and Hun Sen) into exile in Vietnam.

China backed Pol Pot. Deng Xiaoping in late 1978 was rebuilding ties with the United States (the 15 December 1978 announcement of normalisation, effective 1 January 1979) and saw a confident Vietnam aligned with the USSR as a strategic threat. Soviet aid to Vietnam had grown after the 1972 Soviet weaponisation of PAVN.

The Vietnamese decision

The Politburo in Hanoi made the decision to invade Cambodia in late 1978. Pre-invasion preparations included the Treaty of Friendship and Cooperation between Vietnam and the USSR (Moscow, 3 November 1978), which obliged the parties to consult in the event of attack. This was the explicit Soviet umbrella against a Chinese response.

The Kampuchean United Front for National Salvation was founded on 2 December 1978 in the Vietnamese-held Cambodian border zone. The Front, led by Heng Samrin (a former Khmer Rouge division commander who had defected from the Eastern Zone purges), provided the political cover for the invasion as a Cambodian internal liberation rather than a Vietnamese conquest.

The invasion

Vietnamese forces invaded Cambodia on 25 December 1978 with around 150,000 troops in 13 divisions. The campaign was conventional and rapid. Khmer Rouge defences collapsed across multiple axes. The Vietnamese armoured columns took:

• Kratie (30 December 1978),
• Stung Treng (31 December),
• Kompong Cham (3 January 1979),
• Phnom Penh (7 January 1979),
• Battambang (9 January),
• Siem Reap (12 January).

Pol Pot fled by helicopter to Battambang on 6 January, then via Thailand to the Thai-Cambodian border zone. The senior Khmer Rouge leadership escaped with around 30,000 to 40,000 fighters into the Cardamom Mountains and the Thai border zone, where they regrouped with Thai, Chinese, and (covertly) US support.

The People's Republic of Kampuchea

The People's Republic of Kampuchea was proclaimed on 8 January 1979. Heng Samrin served as President of the People's Revolutionary Council; Pen Sovan was Prime Minister, later replaced by Chan Sy and then Hun Sen (Prime Minister from 1985 onwards). Vietnam stationed around 200,000 troops in Cambodia. Vietnamese advisers ran the ministries.

The PRK opened schools, restored Buddhism in a limited form, brought back markets, currency, and family life. The S-21 prison was discovered on 7 January and preserved as the Tuol Sleng Genocide Museum. The mass grave at Choeung Ek was excavated. The Cambodian death toll began to be documented.

The PRK was not internationally recognised. The United Nations General Assembly voted year after year to retain the Khmer Rouge in the Cambodian seat (under the Coalition Government of Democratic Kampuchea from 1982). The ASEAN states, the United States, and China supported the resistance. Australia, after some debate, was the first Western country to recognise the State of Cambodia (the PRK successor) in 1991.

The Sino-Vietnamese war

Deng Xiaoping, on his February 1979 visit to the United States, told Carter that he would "teach Vietnam a lesson". The PRC launched its punitive invasion on 17 February 1979.

Around 250,000 PLA troops, organised in 29 divisions, attacked along a 480-kilometre front into Lang Son, Cao Bang, Lao Cai, and Mong Cai Provinces. The PAVN garrison was largely militia (most regulars were in Cambodia). The PLA used massed infantry and artillery without close air support; older tactics from the 1950s were exposed against PAVN's modern Soviet equipment and battle experience.

The PLA took Cao Bang (24 February), Lao Cai (24 February), and Lang Son (5 March). Both sides suffered heavy losses. China declared the lesson taught and began withdrawing on 6 March; the last PLA units crossed back into China on 16 March 1979.

Casualty estimates vary widely. Plausible figures: around 26,000 PLA killed and 37,000 wounded; around 30,000 PAVN killed and around 100,000 civilian casualties. The PLA destroyed infrastructure on the retreat. Border clashes continued through the 1980s, particularly around Vi Xuyen in 1984 to 1989. PRC border claims were not finally settled until the Treaty on the Land Border between China and Vietnam of 30 December 1999.

The Cambodian aftermath

The Cambodian civil war continued through the 1980s. The Coalition Government of Democratic Kampuchea (CGDK), formed on 22 June 1982, comprised:

• the Khmer Rouge under Khieu Samphan (largest force, around 35,000),
• the National United Front for an Independent, Neutral, Peaceful and Cooperative Cambodia (FUNCINPEC) under Sihanouk,
• the Khmer People's National Liberation Front under Son Sann.

The CGDK fought from the Thai border with US (covert), Chinese, and Thai support. Vietnam withdrew its forces in September 1989 under cost pressure (the war absorbed around 50 per cent of Vietnamese government spending) and Soviet pressure to align with Gorbachev's reform agenda.

The Paris Peace Agreements of 23 October 1991, brokered by the Permanent Five of the Security Council, ended the Cambodian conflict. The UN Transitional Authority in Cambodia (UNTAC, 1992 to 1993) organised the 1993 elections. The Kingdom of Cambodia was restored on 24 September 1993 with Sihanouk as King.

The Khmer Rouge formally dissolved in 1999. Pol Pot died in custody at Anlong Veng on 15 April 1998. The Extraordinary Chambers in the Courts of Cambodia (ECCC), the Khmer Rouge tribunal, convicted Comrade Duch (Case 001, 26 July 2010) and Khieu Samphan and Nuon Chea (Case 002, 7 August 2014 and 16 November 2018) of crimes against humanity and genocide.

Historiography

Stephen Morris (Why Vietnam Invaded Cambodia, 1999) on the politics of the 1978 decision.

Evan Gottesman (Cambodia after the Khmer Rouge, 2003) on the PRK.

Edward O'Dowd (Chinese Military Strategy in the Third Indochina War, 2007) on the Sino-Vietnamese war.

Sophie Quinn-Judge and Christopher Goscha on Vietnamese diplomacy in this phase.

Common exam traps

Treating 1979 as the end of the Cambodian conflict. The civil war continued through the 1980s and ended formally only in 1991 to 1993.

Missing the Soviet alignment. The 3 November 1978 Treaty was the precondition for Vietnam's decision to invade.

Misdating the Chinese invasion. 17 February 1979, not at the time of the Cambodian invasion.

In one sentence

The Vietnamese invasion of Cambodia on 25 December 1978, enabled by the 3 November 1978 Treaty of Friendship with the Soviet Union, ended the Khmer Rouge regime by taking Phnom Penh on 7 January 1979 and installing the People's Republic of Kampuchea under Heng Samrin, and the Chinese punitive invasion of northern Vietnam from 17 February to 16 March 1979 closed the active phase of the Indochina conflict, though the Cambodian civil war ran on until the Paris Peace Agreements of 23 October 1991.