The Cambodian Civil War (, UNGEGN: ) was a civil war in Cambodia fought between the Khmer Rouge, supported by North Vietnam and China, against the government of the Kingdom of Cambodia and, after October 1970, the Khmer Republic, which had succeeded the kingdom after a coup, both supported by the United States and South Vietnam. The conflict was part of the Vietnam War.
The conflict was linked to 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. 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 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.
Setting the stage (1965–1970)
Background
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. 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.
thumb|250px|Meeting in Beijing: [[Mao Zedong (left), Prince Sihanouk (center), and Liu Shaoqi (right)]]
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." 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.