From the 1890s, France controlled Indochina. After World War II, Ho Chi Minh ruled in the north, while France held power in the south. The First Indochina War ended with the defeat of French forces at Dien Bien Phu in 1954. Elections to unite Vietnam were due to take place in July 1955, but they were cancelled. Conflict began between South Vietnam, led by the Ngo Dinh Diem government, and Ho Chi Minh's North Vietnam. Neither side could gain control, despite American advisers and equipment being supplied to South Vietnam. In the Gulf of Tonkin, two American warships, the 'Maddox' and the 'Turner Joy' were allegedly attacked by North Vietnamese torpedo boats. This incident prompted the US Congress to give President Johnson the power to conduct war against North Vietnam. American troop numbers increased, and the 'Rolling Thunder' aerial bombing campaign began. The Tet Offensive in 1968 was a major turning point in the war, with communist forces gaining a strategic victory despite heavy casualties. The anti-war movement in America gained momentum, and the US invasion of Cambodia led to the massacre of anti-war protesters at Kent State University. The Paris Peace Accords were signed in 1973, and two years later North Vietnamese forces captured Saigon, ending the conflict. In 1976 Vietnam was finally unified.