The My Lai massacre, as reported by a South Vietnamese army lieutenant to his superiors, was an "atrocious" incident of revenge. US forces deliberately murdered 400 civilians, shortly after a firefight with Viet Cong troops who had mingled with the villagers on March 16, 1968.
American Lieutenant William Calley was convicted of premeditated murder in ordering the shootings and initially sentenced to life in prison. His sentence was later reduced to 20 years, and in 1971 President Richard Nixon in 1971 ordered him released from prison and placed under house arrest. Finally a federal judge threw out all charges against William Calley and ordered him freed. (source)