During the 1960s, the North Vietnamese and Viet Cong enjoyed sanctuaries in Cambodia, crossing the border for attacks into South Vietnam and then fleeing back across the border to safety. Moreover, the North Vietnamese, with permission of the "neutral" Cambodian government, used Cambodian ports and roads to supply large amounts of materiel for the communist war effort in South Vietnam.
Through the 1960s, military experts (including ex-President Dwight Eisenhower) called for large-scale offensives against the enemy sanctuaries in Cambodia. But, for political reasons, the U.S. only conducted covert operations and limited bombing against supply lines just inside the Cambodian border.
In 1969, when the enemy launched another nationwide offensive in South Vietnam, President Richard Nixon ordered a secret B-52 bombing offensive against the Cambodian sanctuaries. During the next 13 months, USAF B-52s flew more than 4,000 sorties against enemy supply lines and bases in Cambodia (one sortie equals one mission by one aircraft).
By April 1970, communist troops had seized large areas of the country, and a newly formed anti-communist Cambodian government called for assistance. USAF and South Vietnamese Air Force (VNAF) aircraft bombed targets in Cambodia, and U.S. and South Vietnamese troops raided across the border to strike at the enemy's sanctuaries.
In 1971 the communists cut the road that led from the major seaport of Kompong Som to the capital Phnom Penh. USAF and U.S. Army aircraft provided aerial escort to river supply convoys, and continued massive aerial support to Cambodian Army ground operations, which nevertheless suffered repeated setbacks. By 1972, the war in Cambodia turned into a stalemate, but U.S. airpower -- including B-52s -- continued to hammer at the enemy.
When local Khmer Rouge communist insurgents threatened Phnom Penh in 1973, the Cambodian government urgently called upon the U.S. for help. The USAF conducted a massive bombing campaign on the outskirts of the capital. Public pressure, however, forced Congress to stop funding these bombings and other military operations in Southeast Asia, and USAF combat strike missions in Cambodia ended in mid-August 1973.
Phnom Penh fell to the Khmer Rouge in 1975. In the following months, Pol Pot's new communist government executed hundreds of thousands of Cambodians.