On April 17, 1975, the Khmer Rouge overthrew the West-aligned government in Cambodia (Cambodian Civil War, separate article), and then turned the country into a communist state. Nearly two weeks later, on April 30, 1975, North Vietnamese forces captured Saigon and ended the West-aligned South Vietnamese government (Vietnam War, separate article) and later merged the two Vietnams into a single state, the Socialist Republic of Vietnam.
In the aftermath of these communist victories, the international community believed that the two Marxist states would establish close relations due to their shared ideological ties. Instead, shortly after achieving their revolutionary victories, fighting began to break out between their forces. These countries’ respective main ethnic groups, the Khmers (Cambodians) and Vietnamese, have a long history of animosity and conflict since the 12 th century, when their ancient feudal monarchies fought over land and resources.
In the 1800s, the Vietnamese Nguyen Dynasty took control of the Cambodian region of the Mekong Delta (present-day southernmost region of Vietnam) after a period of settlement by ethnic Vietnamese. As well, the Vietnamese conquerors in Cambodia tried to replace the Indian-influenced Khmer culture of the Cambodians with their own Chinese-influenced Vietnamese culture.
During the period 1887-1893, France gained control of the Indochina region, imposing direct rule or entering into protectorate treaties that virtually turned into colonies the territories of Cambodia, Vietnam, and Laos (which were collectively called French Indochina). Thereafter, the Cambodians and Vietnamese turned their nationalist struggles against the French, sometimes forming alliances to defeat and expel their common enemy. Even so, Cambodians continued to harbor a mistrust of the Vietnamese – which would become a major cause of the Cambodian-Vietnamese War.
The revolutionary movements that eventually prevailed in Vietnam and Cambodia (as well as in Laos) trace their origin to 1930 when the Vietnamese Communist Party (VCP) was formed. VCP soon reorganized itself into the Indochinese Communist Party (ICP) to include membership to Cambodian and Laotian communists into the Vietnamese-dominated movement. The great majority of ICP Khmers were not indigenous to Cambodia; rather they consisted mostly of ethnic Khmers who were native to southern Vietnam, and ethnic Vietnamese living in Cambodia.
In 1951, the ICP split itself into three nationalist organizations for Vietnam, Cambodia, and Laos respectively, i. e. Workers Party of Vietnam, Khmer People’s Revolutionary Party (KPRP), and Neo Lao Issara. In December 1946, the Viet Minh (or League for the Independence of Vietnam), a Vietnamese nationalist group that was formed in World War II to fight the Japanese, began an independence war against French rule (First Indochina War, separate article). The Viet Minh prevailed in July 1954. The 1954 Geneva Accords, which ended the war, divided Vietnam into two military zones, which became socialist North Vietnam and West-aligned South Vietnam. War soon broke out between the two Vietnams, with North Vietnam supported by China and the Soviet Union; and South Vietnam supported by the United States. This Cold War conflict, called the Vietnam War (separate article) and which included direct American military involvement in 1965-1970, ended in April 1975 with a North Vietnamese victory. As a result, the two Vietnams were reunified, in July 1976.
Meanwhile in Cambodia, the local revolutionary struggle ended with the 1954 Geneva Accords, which gave the country, led by King Sihanouk, full independence from France. The Accords also ended both French rule and French Indochina, and independence also was granted to Laos and Vietnam. Following the First Indochina War, most of the Khmer communists moved into exile in North Vietnam, while those who remained in Cambodia formed the Pracheachon Party, which participated in the 1955 and 1958 elections. However, government repression forced Pracheachon Party members to go into hiding in the early 1960s.
By the late 1950s, the Cambodian communist movement experienced a resurgence that was spurred by a new generation of young, Paris-education communists who had returned to the country. In September 1960, ICP veteran communists and the new batch of communists met and elected a Central Committee, and renamed the KPRP (Kampuchean People’s Revolutionary Party) as the Worker’s Party of Kampuchea (WPK).
In February 1963, following another government suppression that led to the arrest of communist leaders, the WPK soon came under the control of the younger communists, led by Saloth Sar (later known as Pol Pot), who sidelined the veteran communists whom they viewed as pro-Vietnamese. In September 1966, the WPK was renamed the Kampuchean Communist Party (KCP).
The KCP and its members, as well its military wing, were called “Khmer Rouge” by the Sihanouk government. In January 1968, the Khmer Rouge launched a revolutionary war against the Sihanouk regime, and after Sihanouk was overthrown in March 1970, against the new Cambodian government. In April 1975, the Khmer Rouge triumphed and took over political power in Cambodia, which it renamed Democratic Kampuchea.
During its revolutionary struggle, the Khmer Rouge obtained support from North Vietnam, particularly through the North Vietnamese Army’s capturing large sections of eastern Cambodia, which it later turned over to its Khmer Rouge allies. But the Khmer Rouge held strong anti-Vietnamese sentiment, and deemed its alliance with North Vietnam only as a temporary expedient to combat a common enemy – the United States in particular, Western capitalism in general. The Cambodian communists’ hostility toward the Vietnamese resulted from the historical domination by Vietnam of Cambodia during the pre-colonial period, and the perception that modern-day Vietnam wanted to dominate the whole Indochina region.
Soon after coming to power, the Khmer Rouge launched one of history’s most astounding social revolutions, forcibly emptying cities, towns, and all urban areas, and sending the entire Cambodian population to the countryside to become peasant workers in agrarian communes under a feudal-type forced labor system. All lands and properties were nationalized, banks, schools, hospitals, and most industries, were shut down. Money was abolished. Government officials and military officers of the previous regime, teachers, doctors, academics, businessmen, professionals, and all persons who had associated with the Western “imperialists”, or were deemed “capitalist” or “counter-revolutionary” were jailed, tortured, and executed. Some 11/2 – 21/2 million people, or 25% of the population, died under the Khmer Rouge regime.
In foreign relations, the Khmer Rouge government isolated itself from the international community, expelling all Western nationals, banning the entry of nearly all foreign media, and closing down all foreign embassies. It did, however, later allow a number of foreign diplomatic missions (from communist countries) to reopen in Phnom Penh. As well, it held a seat in the United Nations (UN).
The Khmer Rouge was fiercely nationalistic and xenophobic, and repressed ethnic minorities, including Chams, Chinese, Laotians, Thais, and especially the Vietnamese. Within a few months, it had expelled the remaining 200,000 ethnic Vietnamese from the country, adding to the 300,000 Vietnamese who had been deported by the previous Cambodian regime.
The apparent communist solidarity between Vietnam and Cambodia was only superficial. As early as 1973 even before they had won their revolutions, the Khmer Rouge regularly ambushed North Vietnamese patrols that had crossed over into Cambodia. In 1974, armed clashes took place between their units. Then in 1975, barely one month after both sides won their revolutions, small-scale fighting was breaking out along their common border. Also that month, Khmer Rouge forces seized Phu Quoc, a Vietnamese island located off the Kampuchean coast. They also occupied the island of Tho Chu. Vietnamese forces quickly recaptured these islands, and retaliated by seizing the Cambodian island of Koh Wai.
In June 1975, on an official visit to Hanoi, Pol Pot sought to forge a treaty of friendship with the Vietnamese government. Although no agreement was signed, Vietnam was encouraged enough to withdraw its forces from Koh Wai in August 1975. However, border skirmishes continued in Cambodia’s northeast regions. Also during this time, the Khmer Rouge government expelled ethnic Vietnamese from Kampuchea.
In 1976, the two countries experienced a period of improved relations. Border fighting diminished, mainly because the Khmer Rouge regime was facing internal struggles at this time. In foreign relations, Cambodia called for Vietnam’s membership to the UN. In turn, Vietnam played down reports of widespread human rights violations occurring in Cambodia, which had come from horrific tales told by Cambodian refugees who had managed to escape into Thailand. Commercial flights between the two capitals, Phnom Penh and Hanoi, opened in September 1976.
In May 1976, Cambodian and Vietnamese delegations met to try and resolve their disputed maritime border along the Brévié Line, a French colonial-era demarcation line, but talks broke down. In earlier and later negotiations, both sides remained firm in their positions, and nothing was resolved. Furthermore, many Cambodians also resented the loss of the historic Cambodian lands that now form the southernmost territory of Vietnam (i. e. the Mekong Delta), although the Khmer Rouge government apparently did not officially claim this territory to be part of Cambodia.
The year 1977 saw a resurgence of fighting. In April of that year, the Khmer Rouge attacked the Vietnamese town of Chau Doc (in An Giang Province), and later in September, in what was a prelude to full-scale war, thousands of Khmer soldiers made incursions in Tay Ninh Province. Hundreds of Vietnamese villagers were killed in the fighting, and also as a result of Khmer Rouge atrocities. Vietnam retaliated by sending its planes that attacked targets in Cambodia. In October 1977, Vietnam sent eight divisions which threw back the invaders. Then with the arrival of 58,000 reinforcements in December 1977, in early January 1978, Vietnamese forces entered Kampuchea’s Svay Rieng Province, where they tried to incite the local population to revolt against the Khmer Rouge government. However, no revolt occurred, and the Vietnamese were forced to withdraw across the border.
Also in 1977, Pol Pot visited Beijing, which strengthened Cambodian-Chinese relations. China soon began sending large quantities of weapons and military hardware to Cambodia. The Cambodia-Vietnam crisis took on the dimension of a proxy conflict between China and the Soviet Union, as the Chinese supported the Khmer Rouge regime while the Soviets backed the Vietnamese, for the same reason, i. e. to gain control of the Indochina region, and bring it under their sphere of influence. While the Khmer Rouge’s motives against Vietnam were based on historical reasons, Vietnam in turn found itself surrounded by hostile forces from the southwest (Cambodia) and from the north (China).
In May 1978, Pol Pot began a party purge in the Eastern Zone, whose cadres he believed were too pro-Vietnamese, and who possessed “Khmer bodies with Vietnamese minds”. The Eastern Zone purge was only the latest in a series of violent purges that had taken place across Kampuchea since 1977. Some 100,000 party members were killed in massacres and executions in the Eastern Zone. Also prior to the purge, Pol Pot boasted that only two million Cambodian troops were needed to defeat Vietnam and eradicate its population of fifty million people.
However, the Eastern Zone purge forced thousands of Khmer Rouge party cadres there to flee to Vietnam, where they were arrested and interned by the Vietnamese Army, and subsequently won over by the Vietnamese government. On December 3, 1978, under Vietnamese sponsorship and direction, these ex-Khmer Rouge Eastern Zone cadres organized themselves as the “Kampuchean United Front for National Salvation (KUFNS), a political/military movement whose aim was to overthrow the Khmer Rouge regime.
In June 1978, Vietnam began preparations for a full-scale offensive of Cambodia. And by November 1978, Vietnamese forces were massing along the southwestern border. The Vietnamese government also had taken the precaution to secure its northern border against a possible Chinese invasion by signing the “Vietnamese-Soviet Treaty of Friendship and Cooperation” in November 1978, which guaranteed Russian military intervention in case of a Chinese attack. To counter the Vietnamese build-up, Kampuchea sent 14 divisions to reinforce its eastern border. China also increased its weapons deliveries to Kampuchea.
Full-scale war began on December 25, 1978. On that day, following a diversionary attack on Kratie in Cambodia’s northwest region, the main attack force of the 120,000-strong Vietnamese forces, supported by 20,000 KUFNS fighters and air, artillery, and armored units, launched a swift offensive into southern Cambodia through Takeo Province. The Khmer Rouge had massed its forces in Svay Rieng Province, where the Pol Pot regime believed the Vietnamese would strike. But Vietnamese forces outflanked Svay Rieng Province.
With the fall of Takeo, the road to Phnom Penh lay open. Vietnamese tanks now sped down the flat countryside to the capital. On January 7, 1979, the Vietnamese Army captured Phnom Penh, and overthrew the Khmer Rouge regime. Pol Pot and his staff, together the bulk of the Khmer Rouge Army, made a strategic withdrawal to the jungle mountains of western Cambodia near the Thai border, where they set up a resistance government.