Master Sergeant Donald Walter Duncan (March 18, 1930 – March 25, 2009) 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 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 testimony to the 1967 Russell Tribunal detailing American war crimes in Vietnam.