U.S. and South Vietnamese forces launch a limited "incursion" into Cambodia. The campaign included 13 major ground operations to clear North Vietnamese sanctuaries 20 miles inside the Cambodian border. Some 50,000 South Vietnamese soldiers and 30,000 U.S. troops were involved, making it the largest operation of the war since Operation Junction City in 1967.
The operation began with South Vietnamese forces attacking into the "Parrot's Beak" area of Cambodia that projects into South Vietnam above the Mekong Delta. During the first two days, an 8,000-man South Vietnamese task force, including two infantry divisions, four ranger battalions, and four armored cavalry squadrons, killed 84 communist soldiers while suffering 16 dead and 157 wounded.
The second stage of the campaign began on May 2 with a series of joint U.S.-South Vietnamese operations. These operations were aimed at clearing communist sanctuaries located in the densely vegetated "Fishhook" area of Cambodia (across the border from South Vietnam, immediately north of Tay Ninh Province and west of Binh Long Province, 70 miles from Saigon). The U.S. 1st Cavalry Division and 11th Armored Cavalry Regiment, along with the South Vietnamese 3rd Airborne Brigade, killed 3,190 communists in the action and captured massive amounts of war booty, including 2,000 individual and crew-served weapons, 300 trucks, and 40 tons of foodstuffs. By the time all U.S. ground forces had departed Cambodia on June 30, the Allied forces had discovered and captured or destroyed 10 times more enemy supplies and equipment than they had captured inside South Vietnam during the entire previous year.
Many intelligence analysts at the time believed that the Cambodian incursion dealt a stunning blow to the communists, driving main force units away from the border and damaging their morale, and in the process buying as much as a year for South Vietnam's survival. However, the incursion gave the antiwar movement in the United States a new rallying point. News of the incursion set off a wave of antiwar demonstrations, including one at Kent State University that resulted in the killing of four students by Army National Guard troops and another at Jackson State in Mississippi that resulted in the shooting of two students when police opened fire on a women's dormitory. The incursion also angered many in Congress, who felt that Nixon was illegally widening the scope of the war; this resulted in a series of congressional resolutions and legislative initiatives that would severely limit the executive power of the president.