In March 1969, in violation of the Geneva Accords and without informing Congress, the Nixon administration began the secret B-52 bombing of North Vietnam's Cambodian bases, the "Menu Series" of raids. Sihanouk did not object to these raids since he had his own worries with the rise of Communist power within Cambodia. The B-52s flew 3,630 sorties over Cambodia during Menu Series using various deceptions and false reports to maintain the fiction that only S. Vietnam was being bombed.
Although the raids were effective in suppressing activity that affected S. Vietnam, the fleeing Communist forces clashed with the Cambodian Army as they retreated from border regions. By that time an estimated 35,000 to 60,000 Communist troops were in Cambodia, located principally in eastern Cambodia adjacent to South Vietnam. The resulting turmoil culminated in a military coup that deposed Sihanouk and brought pro-U.S. General Lon Nol to power in Cambodia on 18 March 1970. It has been suggested that the U.S. backed the coup, but there is no evidence of U.S. intervention.
Feeling forced to act, both to support Lon Nol and to reduce the threat to Vietnamization, on 29 April 1970, with U.S. air and logistic support, South Vietnamese forces attacked Communist forces just across the border in the "parrot’s beak" area of Cambodia. On 1 May, US and ARVN forces entered the "fishhook" region. The Cambodian Incursion, named Operation Toan Thang 43, was led by the US 1st Cavalry Division, 11th Armored Cavalry Regiment, and 25th Infantry Division, along with the ARVN units. In both "fishhook" and "parrot’s beak," the main NVA forces were warned by the earlier raids, and withdrew without decisive contact. Large quantities of supplies and major base camp areas were captured and Communist logistics were disrupted, more so than had been anticipated.
On 9 May, a fifty-ship combined U.S.-Vietnamese naval force under Sealords entered Cambodia with the objective of clearing the Mekong River all the way to Phnom Penh. The force, augmented by air cover, destroyed all Communist craft that were met. South Vietnamese forces reached Phnom Penh while the U.S. units were politically constrained to an area south of the Neak Luong transit point which was successfully assaulted and cleared. S. Vietnamese units remained in the area, permanently disrupting Communist logistics and base camps.
Pres. Nixon placed Gen. Abrams under strict rules of engagement designed to keep casualties low and to limit the scope of the operation. A line was drawn 19 miles inside Cambodia and U.S. forces could not advance past it, allowing NVA units who reached the limit to escape unscathed. A time limit was also imposed -- all U.S. forces out of Cambodia by 30 June, publicly announced by Pres. Nixon on 7 May. These limits were strictly observed. South Vietnamese forces continued the fight and in fact remained in Cambodia for the next 18 months.
By the end of June U.S. and ARVN forces in Cambodia had captured or destroyed almost ten thousand tons of materiel and food. In terms of enemy needs this amount was enough rice to feed more than 25,000 troops a full ration for an entire year; individual weapons to equip 55 full-strength battalions; crew-served weapons to equip 33 full battalions; and mortar, rocket, and recoilless rifle ammunition for more than 9,000 average attacks. In all, 11,362 enemy soldiers were killed and over 2,000 captured. ARVN units involved in Cambodia fought well and were often led well. ARVN units returned to S. Vietnam to resume pacification of the Mekong Delta, a goal which had suddenly come much closer to realization. In general in S. Vietnam, large VC and NVA attacks almost ceased for over a year because of the setbacks imposed on them in Cambodia.
In the United States, the aftermath of the 1970 Cambodian Incursion was severely negative. War protests reached new peaks over fears that Nixon was widening the war instead of winding it down. The tragic shooting of students at Kent State University in Ohio occurred 4 May 1970 during a protest over Cambodia. In Cambodia itself, the retreating NVA conquered much of the northeastern sector of the country and turned it over to the Khmer Rouge, thereby strengthening the Communist insurgency.
President Nixon, under pressure from Congress and weakened by the Watergate scandal, ordered a halt to US military action in Southeast Asia after 15 August 1973, as required by the 19 June 1973 Case-Church Amendment. By then, the air strikes had dropped an unprecedented tonnage of bombs and forced the Khmer Rouge to retreat. Cambodian casualties, many innocent civilians, were estimated as high as 100,000 and the Cambodian economy was permanently disrupted. Lon Nol was able to hang on in Phnom Penh, but the bombing devastation turned the country into a basket case. It took the Khmer Rouge until early 1975 to recover enough strength to try again to take Phnom Penh, this time confident that the U.S. would not intervene.
On 1 April 1975, President Lon Nol resigned and left the country. On 12 April 1975 the last American operation in Cambodia, Eagle Pull, was the helicopter evacuation rescue of 82 U.S., 159 Cambodian, and 35 other nationals as Communist forces closed on the capital city. The Communists under Pol Pot took over Phnom Penh and began a killing spree against all known and suspected enemies. At least 1.5 million Cambodians died from execution, forced hardships, or starvation during the Khmer Rouge regime.