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.

A war crime is a violation of the laws of war that gives rise to individual criminal responsibility for actions by combatants in action, such as intentionally killing civilians or intentionally killing prisoners of war, torture, taking hostages, unnecessarily destroying civilian property, deception by perfidy, wartime sexual violence, pillaging, and for any individual that is part of the command structure who orders any attempt to committing mass killings including genocide or ethnic cleansing, the granting of no quarter despite surrender, the conscription of children in the military and flouting the legal distinctions of proportionality and military necessity.