Ronald Lee Ridenhour was an American known for having played a central role in spurring the federal investigation of the 1968 Mỹ Lai massacre in Vietnam. When he first learned of events there, he was serving in the United States 11th Infantry Brigade in Vietnam. He gathered evidence and interviewed people before the end of his tour. After returning to the US in 1969, he wrote to President Nixon, members of his cabinet and two dozen Congressmen recounting what he had learned. A full-scale Department of Defense investigation eventually took place.

The My Lai massacre was a war crime committed by United States Army personnel on 16 March 1968, involving the mass murder of unarmed civilians in Sơn Tịnh district, South Vietnam, during the Vietnam War. Between 347 and 504 civilians were killed by U.S. soldiers from Company C, 1st Battalion, 20th Infantry Regiment and Company B, 4th Battalion, 3rd Infantry Regiment, 11th Brigade, 23rd (Americal) Infantry Division. Victims included men, women, children, and infants. Some of the women were gang-raped and their bodies mutilated, and some soldiers mutilated and raped children who were as young as 12. It is the largest publicized massacre of civilians by U.S. forces in the 20th century.