President Richard Nixon announces that he has ordered the mining of major North Vietnamese ports, as well as other measures, to prevent the flow of arms and material to the communist forces that had invaded South Vietnam in March. Nixon said that foreign ships in North Vietnamese ports would have three days to leave before the mines were activated; U.S. Navy ships would then search or seize ships, and Allied forces would bomb rail lines from China and take whatever other measures were necessary to stem the flow of material. Nixon warned that these actions would stop only when all U.S. prisoners of war were returned and an internationally supervised cease-fire was initiated. If these conditions were met, the United States would "stop all acts of force throughout Indochina and proceed with the complete withdrawal of all forces within four months."
Nixon's action was in response to the North Vietnamese Nguyen Hue Offensive. On March 30, the North Vietnamese had initiated a massive invasion of South Vietnam. Committing almost their entire army to the offensive, the North Vietnamese launched a three-pronged attack. In the initial attack, four North Vietnamese divisions attacked directly across the Demilitarized Zone into Quang Tri province. Following that assault, the North Vietnamese launched two more major attacks: at An Loc in Binh Long Province, 60 miles north of Saigon; and at Kontum in the Central Highlands. With the three attacks, the North Vietnamese committed 500 tanks and 150,000 regular troops (as well as thousands of Viet Cong) supported by heavy rocket and artillery fire. The North Vietnamese, enjoying much success on the battlefield, did not respond to Nixon's demands.
The announcement that North Vietnamese harbors would be mined led to a wave of antiwar demonstrations at home, which resulted in violent clashes with police and 1,800 arrests on college campuses and in cities from Boston to San Jose, California. Police used wooden bullets and tear gas in Berkeley; three police officers were shot in Madison, Wisconsin; and 715 National Guardsmen were activated to quell violence in Minneapolis.