After the Vietnamese invasion of Cambodia in 1978 and the subsequent collapse of Pol Pot's Democratic Kampuchea regime in 1979, the Khmer Rouge, responsible for the Cambodian genocide, fled into the border regions of Thailand. With assistance from China, Pol Pot's remaining forces regrouped and reorganized in the forested and mountainous zones along the Cambodia–Thailand border.

Throughout the 1980s and early 1990s, Khmer Rouge units operated from within refugee camps situated inside Thai territory, launching cross-border attacks in an effort to destabilize the pro-Hanoi People's Republic of Kampuchea. The Thai government, which refused to recognize the Vietnamese-backed regime in Phnom Penh, tacitly supported anti-Vietnamese resistance movements, including the Khmer Rouge.

This period saw heightened tensions between Thailand and Vietnam, marked by frequent Vietnamese incursions and artillery shelling into Thai territory in pursuit of Cambodian guerrillas who continued to harass Vietnamese occupation forces.

Causes

Thailand's suspicion of Vietnamese long-term objectives and fear of Vietnamese support for an internal Thai communist insurgency movement led the Thai government to support United States objectives in South Vietnam during the Vietnam War.

In 1973 a new civilian government in Thailand created a chance for some degree of reconciliation with North Vietnam, when it proposed to remove United States military forces from Thai soil and adopt a more neutralist stance. Hanoi responded by sending a delegation to Bangkok, but talks broke down before any progress in improving relations could be made. Discussions resumed in August 1976, after Hanoi had defeated the South Vietnamese and united the country under its rule. This resulted in a call for an exchange of ambassadors and for an opening of negotiations on trade and economic co-operation, but a military coup in October 1976 ushered in a new Thai government less sympathetic to the Vietnamese communists. Contact was resumed briefly in May 1977, when Vietnam, Thailand, and Laos held a conference to discuss resuming work on the Mekong Development Project, a major cooperative effort that had been halted by the Vietnam War. Beginning in December 1978, however, the conflict in Cambodia dominated diplomatic exchanges, and seasonal Vietnamese military offensives that included incursions across the Thai border and numerous Thai casualties particularly strained the relationship.

In 1979, in retaliation for constant border raids on Vietnam’s border, Bangkok allied itself with the genocidal Khmer Rouge, an adversary of Vietnam, and looked to Beijing for security assistance. Thailand's actions hardened Hanoi's attitude toward Bangkok. As the ASEAN member most vulnerable to a hypothetical Vietnamese attack for having given shelter to the Khmer Rouge in camps within its territory,Puangthong Rungswasdisab, Thailand's Response to the Cambodian Genocide Thailand was foremost among the ASEAN partners opposing Vietnam's 1978 invasion of Cambodia.

Refugee camps on the Cambodia-Thailand border allowed the growth of several anti-Vietnamese guerrilla organizations dedicated to regaining power in Cambodia. In addition to the Khmer Rouge armed forces (known as the NADK), the MOULINAKA, the KPNLF and its armed forces, the KPNLAF, and the Armée Nationale Sihanoukiste supporting Norodom Sihanouk all recruited and trained troops in the border refugee camps, striking at Vietnamese military targets. Because Thailand was offering protection to these groups, the Vietnamese felt justified in attacking Thai military units as well as the refugee camps, which they shelled and assaulted with disregard to the safety of civilian refugees and international humanitarian aid workers.Shawcross W. The Quality of Mercy: Cambodia, Holocaust, and Modern Conscience. New York: Simon and Schuster, 1984 Between 1986 and 1989, the Vietnamese enacted the K5 Plan, a massive network of trenches, wire fences, and minefields along the length of the Cambodia-Thailand border.

Timeline

1979 April: The first group of Vietnamese soldiers appeared on Thai territory next to the irrigation canal near Nong Chan to set up camp overnight. They were told by a Vietnamese speaking Thai Army military intelligence master sergeant to move out of Thailand territory. October: A major offensive by the Vietnamese against Khmer Rouge hideouts in their mountain sanctuaries pushed thousands of Khmer Rouge soldiers, their families, and the civilians under their control to the Thai border. 8 November: Thai artillery fire hit Nong Chan Refugee Camp, when a Thai soldier was accused of raping a Khmer woman and was shot to death. The Thai military commander Colonel Prachak Sawaengchit ordered his troops to shell Nong ChanPilger, John, Heroes, South End Press, Cambridge, MA, 2001, p. 421., killing about 100 refugees.Burgess, John "Thais Blamed in Shelling Deaths at Refugee Camp," The Washington Post, November 9, 1979, p. 1. 12 November: Vietnamese attacks opposite Ban Laem drove 5,000 Khmer Rouge troops and villagers into Thailand. About half went to Kamput Holding Center.

1980 23 June: In response to the organised repatriation of thousands of refugees, 200 People's Army of Vietnam (PAVN) troops crossed the border at 02:00 into the Ban Non Mak Mun area, including Nong Chan Refugee Camp, setting off a three-day artillery battle that left about 200 dead, including around 22–130 Thai soldiers, one Thai villager, scores of refugees and approximately 72–100 Vietnamese troops. Hundreds of refugees were reported killed, many by a Thai artillery barrage that struck one of the camps. Others were caught in the crossfire.Henry Kamm, New York Times, 24 June 1980, cites Associated Press figures of 130 Thai soldiers, 72 Vietnamese, and up to 400 refugees killed; Time Magazine, 7 July 1980 claims 22 Thais killed and 100 Vietnamese. Several hundred refugees who resisted the Vietnamese were bound and executed.Mason L, Brown R. Rice, Rivalry and Politics: Managing Cambodian Relief. Notre Dame: University of Notre Dame Press, 1983, p. 76. Vietnamese troops temporarily seized two Thai border villages including Ban Non Mak Mun and shelled others. 24 June: Still controlling Nong Chan, Vietnamese forces fought artillery and small arms duels with Thai troops and attacked guerrilla strongpoints. The Vietnamese shot down two Thai military aircraft. 26 June: Vietnamese troops seized two relief officials (Robert Ashe and International Committee of the Red Cross (ICRC) Medical Coordinator Dr. Pierre Perrin) and two American photographers at Nong Chan Refugee Camp.Vietnam Said to Seize Photographers and Relief Workers at Thai Border; U.S. Envoy Asks Halt Border Is Reported Crossed Repatriation Is Denounced, 27 June 1980, Friday; Page A6, 647 words

1981 4 January: Vietnamese forces stormed across the border, opened fire with rocket-propelled grenades and automatic weapons, and battled with Thai troops before being pushed back. Two Thai soldiers were killed and one was wounded during the early morning, 90-minute battle. Between 50 and 60 Vietnamese soldiers reportedly opened fire on a Thai patrol half a mile inside Thailand. Vietnamese casualties are unknown.Thais Report 2 Troops Slain Repelling Vietnamese Force, UPI, Published: 4 January 1981 Thai troop reinforcements were rushed to the border the next day and put on alert against another cross-border raid by Hanoi's troops.Thai Troops Placed on Alert After Raid by Vietnamese, UPI, Published: Monday, 5 January 1981

1982 11 February: Vietnam People's Air Force An-26 26264 was shot down by two Royal Thai Air Force Northrop F-5Es and crashed in a rice field near Prachinburi, Thailand during an intelligence-gathering mission from Phnom Penh, reportedly killing one of 13 onboard Early March: A spate of incidents along the border, culminating in the intrusion of 300 Vietnamese troops and the killing of a number of Thai Border Patrol Police. 21 October: Vietnamese gunners opened fire on a Thai reconnaissance plane near the border, but did not hit the aircraft. The plane returned to its base inside Thailand.Vietnam Attacks Thai Plane, UPI, Published: 21 October 1982

1983 January: Cambodian government troops, backed by Vietnamese units, conducted a major offensive against the Coalition Government of Democratic Kampuchea, which united three resistance factions. The fighting spilled over onto Thai soil. More than 47,000 Cambodians fled to Thailand. 16 January: Vietnamese troops recaptured the hamlet of Yeang Dangkum, east of Nong Chan. Insurgents from the non-Communist Khmer People's National Liberation Front (KPNLF) captured the hamlet on 26 December and held it as part of a series of initiatives at year's end. 21 January: Vietnamese artillery attack forced the KPNLF base in the 0'Bok pass to move into Thailand. Non-combatants return at the end of the month. 31 January – 1 February: With heavy artillery support, 4,000 armour-led Vietnamese troops launched an assault against Nong Chan, one of the largest refugee camps on the border, destroying it. Ground fighting was reported outside the camp between Vietnamese troops based in Cambodia and about 2,000 KPNLF guerrillas.Van der Kroeff, J. "Refugees and Rebels: Dimensions of the Thai-Kampuchean Border Conflict", Asian Affairs, Vol. 10, No. 1 (Spring. 1983), pp. 19–36. At the same time the Vietnamese kept up a steady barrage of shells, rockets and mortars. At least 50 shells landed in Thai territory, killing a 66-year-old farmer and damaging several houses and a Buddhist temple."Vietnam Troops Attack Cambodia Refugee Camp", Reuters, Tuesday, 1 February 1983. The refugee population of about 24,000 fled with unknown casualties while MOULINAKA units were brushed aside, and KPNLF forces withdrew after a 36-hour fight. The Khao-I-Dang ICRC hospital received over 100 civilian wounded. 31 March: 1,000 Vietnamese troops, augmented by about 600 PRK "people's volunteers," attacked the Khmer Rouge refugee settlements of Phnom Chat and Chamkar Kor, aided by artillery, rocket, and Soviet T-54 tank fire. Thai officials reported that the Vietnamese attacks again had resulted in "spillovers" with Vietnamese artillery and mortar shells falling into adjacent Thai territory.The Bangkok Post, 1 April 1983, quoted in Justus van der Kroef, "Kampuchea: Southeast Asia's Flashpoint", Parameters, Journal of the US Army War College, Spring 1984, p. 62. Vietnamese troops clashed with Thai forces for several days, drawing Bangkok into a defensive campaign. Intense exchange of artillery and tank fire killed 30 civilians and injured some 300 persons. Approximately 22,000 Cambodian civilians fled to Thailand for refuge. Early April: PVNA destroyed the camp of Phnom Chat, civilians evacuated to Red Hill. Sihanouk's Camp David was attacked and civilians moved to Green Hill. One Thai jet was shot down. 1 April: The Thai Government reported that a large force of Vietnamese troops had attacked a total of 3 refugee camps along the Thai-Cambodian border. The New York Times reported that at least 7,000 civilians had fled westward into Thai territory. 3 April: At least 100 Vietnamese troops crossed into Thailand and fought hand-to-hand with a Thai border patrol, killing five Thai soldiers and wounding eight."Vietnamese Battle Thais", AP, Sunday, 3 April 1983 An assault on Ampil Camp, the KPNLF headquarters, failed because KPNLF sabotage units had blown up several fuel depots in the weeks prior to the attack, leaving the division short of diesel and unable to mobilise its armour.Kenneth Conboy, The NVA and Viet Cong, (1992) p. 29.