From Wikipedia

Mai Lai

The My Lai Massacre was the mass murder conducted by a unit of the U.S. Army on March 16, 1968 of 347–504 unarmed citizens in South Vietnam, all of whom were civilians and a majority of whom were women, children (including babies) and elderly people.
When the incident became public knowledge in 1969, it prompted widespread outrage around the world. The massacre also increased domestic opposition to the U.S. involvement in the Vietnam War. Three U.S. servicemen who made an effort to halt the massacre and protect the wounded were denounced by U.S. Congressmen, received hate mail and death threats and found mutilated animals on their doorsteps. Only 30 years after the event were their efforts honored.