The Khmer republic was officially declared on October 9, 1970. This republic can be described as a government led by the military and was based on pro-US thoughts. This right wing government was headed by two men; general Lon Nol and prince Sisowath Sirik Matak. They took the power after a coup against prince Norodom Sihanouk on March 18, 1970. He had been head of state of the country until then.
Sirik Matak had been the Prime Minister during the first year of the Republic. Lon Nol fell ill and when he returned from his stay at a hospital he resigned during a cabinet crisis. This caused the government to dissolve and on 16 October 1971 Lon Nol took away the power of the National Assembly. He ordered fro a new constitution to be written as the National Assembly had no legislative power anymore and claimed that having a new constitution had to be written to prevent anarchy. This action was followed by a protest by 400 Buddhist monks.
The reason for the coup in March 1970 was the fact that Sihanouk had tolerated North Vietnamese activity inside the borders of Cambodia. This resulted in armed Vietnamese communists taking control over large parts of eastern Cambodia, which of course created a lot of unrest. Another reason for the coup was Sihanouks policy of pursuing a neutral position with his anti-American policies. Due to this policy the Cambodian economy suffered greatly.
The Kingdom of Cambodia turned into a republic after Sihanouks reign had ended (even though officially the throne had not been occupied officially since the death of king Norodom Suramarit). The intention and character of the new reign over Cambodia was very different. The new regime was based on nationalist politics and could be called right-wing. The period of time during which Sihanouk had cooperated with the North Vietnamese regime as well as with the Viet Cong came to an end. This caused Cambodia to align itself with South Vietnam and the second Indochina war that was taking place at this time.
Inside the Cambodian border the Khmer republic was met by the FUNK ( Front Uni National du Kampuchea). FUNK was made up of an alliance between supporters of Sihanouk and the Kampuchea Communist Party. The Cambodian People's National Liberation Armed Forces (CPNLAF), backed by the People's Army of Vietnam and the Viet Cong (also known as the National Liberation Front) conducted the insurgency and managed to occupy areas of Cambodia as part of the war with South Vietnam. Eventually the army of the Khmer republic was defeated by both the CPNLAF and the Vietnamese forces. The republic fell on 17 April 1975. The reason for this was the fact that the Khmer republic, backed by military and financial help from the US, was not strong enough to beat its enemies due to a lack of training. After the republic finally fell Phnom Penh was taken over by Cambodian communists.