For most of the 1960s eastern Cambodia was the back room of the war next door. North Vietnamese units had been crossing since 1963, and in 1966 Prince Norodom Sihanouk — hedging against a communist victory he thought inevitable — agreed with Beijing to permit permanent bases on Cambodian soil and resupply through Sihanoukville. Weapons came up the Sihanouk Trail to base areas where roughly 40,000 North Vietnamese and Viet Cong troops could rest and reorganise beyond American reach. Washington knew. Rather than confront Sihanouk, President Johnson authorised covert reconnaissance across the border, and after the Tet offensive of 1969 an enraged Nixon signed off on a secret bombing campaign. Cambodia was, formally, neutral.

The arrangement collapsed in weeks. Sihanouk was deposed in March and replaced by General Lon Nol, pro-American and militarily weak. On 29 March the North Vietnamese attacked the Cambodian army — at the explicit request of the Khmer Rouge, Soviet archives later revealed — and swept up much of the east of the country. On 14 April Lon Nol appealed for help, saying Cambodia was about to lose its independence. The communist leadership, meanwhile, was already walking north; Trương Như Tảng, justice minister in the Provisional Revolutionary Government, remembered the retreat toward Kratié as forced marches broken up by B-52 raids. Nixon was blindsided by all of it. "What the hell do those clowns do out there in Langley?" he asked the National Security Council.

Roughly 400,000 people of Vietnamese descent lived in Cambodia, many in families settled for generations. Lon Nol decided they were useful as hostages against North Vietnamese attack and had them rounded up and interned. Soldiers and civilians went further, and the police helped organise it: thousands were murdered in pogroms across the country. On 15 April, at the village of Chroy Changvar, 800 Vietnamese men were tied together, executed, and pushed into the Mekong. The bodies drifted downstream and came ashore in South Vietnam, where their arrival enraged a public on both sides of a war otherwise divided about everything. These were fishermen, shopkeepers, farmers and their sons. They had no part in the fighting that was used to justify killing them.

The incursion came in two thrusts. On 30 April some 8,700 South Vietnamese troops under Lieutenant General Đỗ Cao Trí — who had refused to start a day earlier because his astrologer found the heavens unfavourable — drove into the Parrot's Beak in Svay Rieng. The next morning 36 B-52s laid 774 tons of bombs on the Fishhook, and Task Force Shoemaker went in behind them with 10,000 American and 5,000 South Vietnamese troops. The only conventional battle Americans fought was at Snuol, where resistance was answered with two days of air strikes that left the town in rubble. Southwest of it they found what soldiers named "The City": two square miles of huts and bunkers packed with supplies, with truck repair shops, hospitals, eighteen mess halls, a pig farm and a swimming pool. The haul ran to 22,892 individual weapons, up to 8,000 tons of rice and 1,800 tons of ammunition.

The allies claimed 11,369 enemy dead and 2,328 prisoners; a CIA official later acknowledged the body count included civilians. American losses were 338 killed, South Vietnamese 638; General Nguyễn Viết Thanh, who planned the Parrot's Beak operation, died in a helicopter crash on 2 May. At home the reaction was immediate and violent. On 4 May, Ohio National Guardsmen shot dead four unarmed students at Kent State University, two of whom were not taking part in the protest. On 15 May police killed two students and wounded twelve at Jackson State College in Mississippi, a historically Black school whose dead received a fraction of the attention. Congress moved too: the Cooper–Church amendment, passed that December, barred American ground troops from Cambodia and Laos, and the Gulf of Tonkin Resolution, under which two presidents had waged seven years of undeclared war, was repealed.

Nixon called it "the most successful military operation of the entire war." The North Vietnamese, forewarned and under orders to break contact rather than fight, rebuilt their logistics through Laos; by 1972 they were supplying the siege of An Loc out of Cambodia again. What did not recover was Cambodia. The allies withdrew from ground they could not hold, the North Vietnamese and the Khmer Rouge filled the vacuum, and an insurgency that had been marginal in 1968 was flooded with weapons, advisers and — historians have argued ever since — recruits the incursion itself produced. The fighting spread west: in January 1971 South Vietnamese troops fought to reopen National Road 4 at the Pich Nil Pass in the Elephant Mountains, closed by a North Vietnamese division. Four years after the incursion the Khmer Rouge held Kampot. Five years after it, they held everything.