The Cambodian Civil War was a military conflict that pitted the forces of the Communist Party of Kampuchea and their allies the Democratic Republic of Vietnam and the Viet Cong against the government forces of the Kingdom of Cambodia and, after October 1970, the Khmer Republic, which were supported by the United States and the Republic of Vietnam.
The struggle was complicated by the influence and actions of the allies of the two warring sides. North Vietnam's People's Army of Vietnam 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 Cambodian coup of 18 March 1970 put a pro-American, anti-North Vietnamese government in power and ended Cambodia's neutrality in the Vietnam War. The PAVN was now threatened by a newly unfriendly Cambodian government.
Between March and June 1970, the North Vietnamese moved many of its military installations further inside Cambodia to protect them from U.S. incursions and bombing, capturing most of the northeastern third of the country in engagements with the Cambodian army.