The Cambodian Civil War was a conflict that pitted the forces of the Communist Party of Kampuchea (known as the Khmer Rouge) and their allies the Democratic Republic of Vietnam and the National Liberation Front (NLF) against the government forces of Cambodia (after October 1970, the Khmer Republic), which were supported by the United States (U.S.) and the Republic of Vietnam.
The struggle was exacerbated by the influence and actions of the allies of the two warring sides. North Vietnamese involvement was designed to protect its base areas and sanctuaries in eastern Cambodia, without which the prosecution of its military effort in South Vietnam would have been more difficult. The U.S. was motivated by the need to buy time for its withdrawal from Southeast Asia and to protect its ally, South Vietnam. American, South Vietnamese, and North Vietnamese forces directly participated (at one time or another) in the fighting. The central government was mainly assisted by the application of massive U.S. aerial bombing campaigns and direct material and financial aid.
The Republican government, after five years of savage fighting (and after suffering massive casualties, the destruction of its economy, the starvation of its population, and grievous atrocities committed by its enemy), was defeated on 17 April 1975. The victorious Khmer Rouge proclaimed the establishment of Democratic Kampuchea.