The Khmer Republic (Khmer: សាធារណរដ្ឋខ្មែរ, République khmère) was the pro–United States military-led republican government of Cambodia that was formally declared on 9 October 1970. Politically, the Khmer Republic was headed by General Lon Nol and Prince Sisowath Sirik Matak that took power in the 18 March 1970 coup against Prince Norodom Sihanouk, then the country's head of state.
The main causes of the coup were Norodom Sihanouk's toleration of North Vietnamese activity within Cambodia's borders, allowing heavily armed Vietnamese Communist outfits de facto control over vast areas of eastern Cambodia. Another important factor was the dire state of the Cambodian economy, an indirect result of Sihanouk's policies of pursuing neutrality.
With the removal of Sihanouk, the existing Kingdom of Cambodia became a republic, although the throne had been officially vacant for some years since the death of King Norodom Suramarit. The character of the new regime was right-wing and nationalist; most significantly, it ended Sihanouk's period of covert co-operation with the North Vietnamese regime and the Viet Cong, and aligned Cambodia with South Vietnam in the ongoing Second Indochina War. The Khmer Republic was opposed within the Cambodian borders by the Front uni national du Kampuchéa (FUNK), a relatively broad alliance between Sihanouk, his supporters, and the Communist Party of Kampuchea. The insurgency itself was conducted by the CPNLAF, the Cambodian People's National Liberation Armed Forces: they were backed by both the People's Army of Vietnam (PAVN) and the National Liberation Front (NLF, better known as the Viet Cong), who occupied parts of Cambodia as part of their ongoing war with the South Vietnamese government.
Despite the strongly militaristic character of the Khmer Republic, and quantities of military and financial aid from the United States, its military the Khmer National Armed Forces (Forces armées nationales khmères, or FANK) was poorly trained and unable to defeat either the CPNLAF or the Vietnamese forces of the PAVN and NLF. The Republic eventually fell on 17 April 1975, when the Khmer Rouge took Phnom Penh and briefly restored the Kingdom of Cambodia before renaming itself Democratic Kampuchea in January 1976.