American frustration at the continued infiltration into South Vietnam following the end of the Rolling Thunder offensive led eventually to the secret bombing of enemy sanctuaries in Cambodia by B-52s.  The first strike took place on 18 March 1969 and by 26 May 1970 the B-52s had flown a total of 4,308 sorties over Cambodia, mostly at night under ground radar control.  In March 1970 Prince Sihanouk was deposed by a military junta led by General Lon Nol who formed the Khmer Republic.  The new government was more sympathetic to the cause of South Vietnam and soon found itself at odds with Hanoi.  The North Vietnamese responded by openly occupying much of eastern Cambodia and attempting to cut off Phnom Penh.  Lon Nol asked for US assistance which gave President Nixon a legitimate excuse to attack the NVA and VC sanctuaries in Cambodia that had been a thorn in the American’s side ever since the war began.

Tactical air strikes commenced on 24 April and five days later 42,000 American and 48,000 ARVN troops began a limited incursion into Cambodia.  Initially at least, the air strikes were limited to 18 miles across the border in support of the ground troops.  Most of the allied troops had been withdrawn back into South Vietnam by 29 June having captured thousands of tons of weapons and supplies and killed or captured many NVA and VC soldiers.  However, although the enemy forces close to the South Vietnamese border had been hit hard, those deeper in Cambodia still threatened the new government.  On 30 June US air strikes commenced against NVA forces west of the Mekong River who were threatening provincial towns close to Phnom Penh.  American air strikes in Cambodia continued for the next three years but by the end of 1970 the North Vietnamese still occupied about half of the country.  Phnom Penh itself was largely cut off and supplies to the capital had to be taken up the Mekong River under heavy air and naval escort.  A number of small-scale ground incursions by ARVN troops in 1971 and 1972 met with varying success.  Air strikes continued after the cease-fire in South Vietnam in January 1973 and in May of that year air power thwarted an attempt by Khmer Rouge rebels to cut off and capture Phnom Penh.  However, the US Congress was by now tired of the war in Southeast Asia and forced President Nixon to cut off funds for the bombing of Cambodia with the result that the last combat missions were flown on 15 August 1973.