Media coverage enabled millions of people to see graphic scenes of human suffering in the Vietnam War. The first major demonstration against the war was a march in New York City in 1965, in which some 25,000 people participated. Two years later mass demonstrations attracted hundreds of thousands of participants in Washington, DC, as well as in London and other European capitals. Many protesters were college students, and they formed anti-US-government groups such as Students for a Democratic Society (SDS) to oppose the war.

The anti-war movement continued after Richard Nixon became president in 1968. Protests against the invasion of Cambodia in 1970 led to more demonstrations, including the student demonstration at Kent State University in Ohio, in which four people were shot dead by the National Guard. In 1971 Lieutenant William Calley, Jr, was convicted of war crimes for his unit's 1968 massacre of some 500 civilians, all of them women, children, and old men, in a village of South Vietnam. This case further fuelled anti-war sentiment. Many people also protested against the environmental damage caused by weedkillers such as Agent Orange, which was later found to be highly poisonous.