Nixon's bold move in Cambodia resulted in the first major international crisis of his administration. Following a military coup in Cambodia that saw the ruler, Prince Sihanouk, ousted, the number of North Vietnamese attacks from Cambodian sanctuaries on U.S. forces in South Vietnam had risen dramatically in the spring of 1970. In response, Nixon ordered more than 30,000 U.S. ground troops into the neutral country to eliminate enemy sanctuaries along the border, disrupt supply lines, and capture the Communist headquarters in Cambodia. Importantly, the incursion followed more than a year of secret bombing of targets in Cambodia which became public knowledge only in May 1970. The ‘widening down’ of the Vietnam War to include attacks on a neutral country led to unprecedented upheaval in the United States, which culminated in the deadly shooting of four students in Kent State University by the National Guard. The controversial invasion of Cambodia also saw the beginning of a congressional process which resulted in the 1973 War Powers Act, which repealed the 1964 Gulf of Tonkin Resolution that had given President Johnson a carte blanche to expand the war in Vietnam.