The My Lai massacre of March 16, 1968, was one of the most tragic incidents resulting from U.S. involvement in Viet Nam. American forces of the 20th Infantry regiment attacked and brutally killed over three hundred civilians, many of them women, small children, even infants. Later evidence indicated that many of the dead were mutilated and some of the women raped before being executed. Although efforts were originally made to disguise the enormity of the massacre, a number of soldiers who had witnessed the attack wrote letters to superior officers who at first did little to investigate. When news of the massacre became public, Lieutenant William Calley, who had commanded the responsible regiment, was court martialed and charged with 24 counts of premeditated murder. He was convicted, and sentenced to life in prison, but only served three and a half years.
Outrage over the massacre was a substantial factor in turning public opinion in the United States against the war.