The Cambodian Civil War (Khmer: សង្គ្រាមស៊ីវិលកម្ពុជា, UNGEGN: Sângkréam Sĭvĭl Kâmpŭchéa) was a civil war in Cambodia fought between the Khmer Rouge against the government of the Kingdom of Cambodia. Following the March 1970 coup, the conflict shifted as the Khmer Rouge and forces loyal to Norodom Sihanouk, now supported by North Vietnam and China, fought against the Khmer Republic, which was supported by the United States and South Vietnam.
The conflict was part of the Vietnam War. The North Vietnamese People's Army of Vietnam (PAVN) was involved to protect its bases in eastern Cambodia, which were crucial to its military effort in South Vietnam. This presence was initially tolerated by Prince Norodom Sihanouk, the Cambodian head of state, but domestic resistance combined with China and North Vietnam aiding the anti-government Khmer Rouge caused him to request help from the Soviet Union to stop this.
In March 1970, Sihanouk was deposed by the Cambodian National Assembly, following wide scale protests in the capital against the PAVN presence in the country. He was replaced by a pro-American government which demanded that the PAVN leave Cambodia. They refused and, at the request of the Khmer Rouge, invaded Cambodia, capturing most of the northeastern third of the country from the Khmer National Armed Forces (FANK) between March and June 1970. They handed over some of the territory to the Khmer Rouge and increased material assistance to the group, thus empowering what was at the time a small guerrilla movement. In response, FANK was expanded to fight the PAVN and growing Khmer Rouge. U.S. involvement was motivated by the desire to buy time for its withdrawal from Southeast Asia, to protect South Vietnam, and to prevent the spread of communism to Cambodia. The U.S. assisted the Khmer government with massive U.S. aerial bombing campaigns and direct material and financial aid, while the PAVN continued to occupy Cambodian territory and occasionally engage the FANK in combat.
The war caused a refugee crisis in Cambodia with two million people—more than 25 percent of the population—displaced from rural areas into the cities, with the capital Phnom Penh's population growing from 600,000 in 1970 to nearly 2 million by 1975. The Cambodian government estimated that more than 20 percent of the property in the country had been destroyed during the war. In total, an estimated 275,000–310,000 people were killed as a result of the war, including 30,000 to 150,000 killed in U.S. bombing campaigns.
On 17 April 1975, after five years of fighting, the Khmer Republic was ultimately defeated, with the victorious Khmer Rouge establishing a new regime that would officially be called Democratic Kampuchea the following year. Once in power, the Khmer Rouge carried out the Cambodian genocide, one of the deadliest in history.
During the early-to-mid-1960s, Prince Norodom Sihanouk's policies had protected his nation from the turmoil that engulfed Laos and South Vietnam. Neither the People's Republic of China (PRC) nor North Vietnam disputed Sihanouk's claim to represent "progressive" political policies and the leadership of the prince's domestic leftist opposition, the Pracheachon Party, had been integrated into the government. On 3 May 1965, Sihanouk broke diplomatic relations with the U.S., ended the flow of American aid, and turned to the PRC and the Soviet Union for economic and military assistance.
By the late 1960s, Sihanouk's delicate domestic and foreign policy balancing act was beginning to go awry. In 1966, an agreement was struck between the prince and the Chinese, allowing the presence of large-scale PAVN and Viet Cong troop deployments and logistical bases in the eastern border regions. He had also agreed to allow the use of the port of Sihanoukville by communist-flagged vessels delivering supplies and material to support the PAVN/Viet Cong military effort in South Vietnam. These concessions made Cambodia's neutrality questionable, which had been guaranteed by the Geneva Conference of 1954.
Sihanouk was convinced that the PRC, not the U.S., would eventually control the Indochinese Peninsula and that "our interests are best served by dealing with the camp that one day will dominate the whole of Asia – and by coming to terms before its victory – in order to obtain the best terms possible."
During the same year, however, he allowed his pro-American minister of defense, General Lon Nol, to crack down on leftist activities, crushing the Pracheachon by accusing its members of subversion and subservience to Hanoi. Simultaneously, Sihanouk lost the support of Cambodia's conservatives as a result of his failure to come to grips with the deteriorating economic situation (exacerbated by the loss of rice exports, most of which went to the PAVN/Viet Cong) and with the growing communist military presence.
On 11 September 1966, Cambodia held its first open election. Through manipulation and harassment the conservative wing of the ruling Sangkum party won 75 percent of the seats in the National Assembly to Sihanouk's surprise. Lon Nol was chosen by the right as prime minister and, as his deputy, they named Prince Sirik Matak; an ultraconservative member of the Sisowath branch of the royal clan and long-time enemy of his cousin Sihanouk. In addition to these developments and the clash of interests among Phnom Penh's politicized elite, social tensions created a favorable environment for the growth of a domestic communist insurgency in the rural areas.
The prince then found himself in a political dilemma. To maintain the balance against the rising tide of the conservatives, he named the leaders of the very group he had been oppressing as members of a "counter-government" that was meant to monitor and criticize Lon Nol's administration. One of Lon Nol's first priorities was to fix the ailing economy by halting the illegal sale of rice to the communists. Soldiers were dispatched to the rice-growing areas to forcibly collect the harvests at gunpoint, and they paid only the low government price. There was widespread unrest, especially in rice-rich Battambang Province, an area long-noted for the presence of large landowners, great disparity in wealth, and where the communists still had some influence.
On 2 April 1967, while Sihanouk was out of the country in France, a rebellion broke out in the area around Samlaut in Battambang, when enraged villagers attacked a tax collection brigade. With the probable encouragement of local communist cadres, the insurrection quickly spread throughout the whole region. Lon Nol, acting in the prince's absence (but with his approval), responded by declaring martial law. Hundreds of peasants were killed and whole villages were laid waste during the repression. After returning home in March, Sihanouk abandoned his centrist position and personally ordered the arrest of Khieu Samphan, Hou Yuon, and Hu Nim, the leaders of the "counter government", all of whom escaped into the northeast.
Simultaneously, Sihanouk ordered the arrest of Chinese middlemen involved in the illegal rice trade, thereby raising government revenues and placating the conservatives. Lon Nol was forced to resign, and, in a typical move, the prince named new leftists to the government to balance the conservatives. The immediate crisis had passed, but it engendered two consequences. First, it drove thousands of new recruits into the arms of the hard-line maquis of the Cambodian Communist Party (which Sihanouk labelled the Khmers rouges ("Red Khmers")). Second, for the peasantry, the name of Lon Nol became associated with ruthless repression throughout Cambodia.
While the 1967 insurgency had been unplanned, the Khmer Rouge tried, without much success, to organize a more serious revolt during the following year. The prince's decimation of the Prachea Chon and the urban communists had, however, cleared the field of competition for Saloth Sar (also known as Pol Pot), Ieng Sary, and Son Sen—the Maoist leadership of the maquisards. They led their followers into the highlands of the northeast and into the lands of the Khmer Loeu, a tribal people who were hostile to both the lowland Khmers and the central government. For the Khmer Rouge, who still lacked assistance from the North Vietnamese, it was a period of regroupment, organization, and training. Hanoi basically ignored its Chinese-sponsored allies, and the indifference of their "fraternal comrades" to their insurgency between 1967 and 1969 would make an indelible impression on the Khmer Rouge leadership.