In 1967, as American casualities in the Vietnam War mounted, and no end to the conflict seemed to be in sight, anti-war demonstrations that had been escalating for several years took on a new size and tone. Rather than being a few hundred or a thousand college students here or there, the new protests, like this one in Washington DC, featured more than 100,000 protestors. Not just students, these protestors included returned Vietnam vets, and celebrities such as boxer Muhammad Ali and pediatrician Dr. Benjamin Spock. Among the Vietnam vets against the war was future Senator and presidential candidate John Kerry.
By 1970, local authorities and the Nixon administration were at their wits' end trying to deal with the overwhelming tide of anti-war sentiment. The May 4, 1970 killing of four unarmed students by the National Guard at Kent State University in Ohio marked a nadir in relations between the protestors (plus innocent passers-by) and the authorities.
Public pressure was so great that President Nixon was forced to pull the last American troops out of Vietnam in August of 1973. South Vietnam held out for 1 1/2 years more, before the April 1975 Fall of Saigon and the communist reunification of Vietnam.