President Richard Nixon gives his formal authorization to commit U.S. combat troops, in cooperation with South Vietnamese units, against communist troop sanctuaries in Cambodia.

Secretary of State William Rogers and Secretary of Defense Melvin Laird, who had continually argued for a downsizing of the U.S. effort in Vietnam, were excluded from the decision to use U.S. troops in Cambodia. Gen. Earle Wheeler, Chairman of the Joint Chiefs of Staff, cabled Gen. Creighton Abrams, senior U.S. commander in Saigon, informing him of the decision that a “higher authority has authorized certain military actions to protect U.S. forces operating in South Vietnam.” Nixon believed that the operation was necessary as a pre-emptive strike to forestall North Vietnamese attacks from Cambodia into South Vietnam as the U.S. forces withdrew and the South Vietnamese assumed more responsibility for the fighting. Nevertheless, three National Security Council staff members and key aides to presidential assistant Henry Kissinger resigned in protest over what amounted to an invasion of Cambodia.

When Nixon publicly announced the Cambodian incursion on April 30, it set off a wave of antiwar demonstrations. A protest at Kent State University resulted in the killing of four students by Army National Guard troops. Another student rally at Jackson State College in Mississippi resulted in the death of two students and 12 wounded when police opened fire on a women’s dormitory. The incursion angered many in Congress, who felt that Nixon was illegally widening the war; this resulted in a series of congressional resolutions and legislative initiatives that would severely limit the executive power of the president.