Master Sergeant Donald Walter Duncan was a U.S. Army Special Forces soldier who served during the Vietnam War, helping to establish the guerrilla infiltration force Project DELTA there. Following his return to the United States, Duncan became one of the earliest military opponents of the war and one of the antiwar movement's leading public figures. Duncan is best remembered as the cover image on the February 1966 issue of Ramparts where he announced "I quit", as well as for his 1967 book The New Legions and his testimony to the 1967 Russell Tribunal, both of which detailed American war crimes in Vietnam.

The Russell Tribunal, also known as the International War Crimes Tribunal, Russell–Sartre Tribunal, or Stockholm Tribunal, was a private People's Tribunal organised in 1966 by Bertrand Russell, British philosopher and Nobel Prize winner, and hosted by French philosopher and writer Jean-Paul Sartre, along with Lelio Basso, Simone de Beauvoir, Vladimir Dedijer, Ralph Schoenman, Isaac Deutscher, Günther Anders and several others. The tribunal investigated and evaluated American foreign policy and military intervention in Vietnam.