The 20th Infantry Regiment is a United States Army infantry regiment. Currently only the 5th Battalion of the 20th Infantry still exists. Stationed at Joint Base Lewis-McChord and part of the 1st Stryker Brigade Combat Team, 2nd Infantry Division, 5-20 Infantry was one of the original battalions selected to take part in the testing and fielding of the U.S. Army's then-new Stryker vehicle. During the Vietnam War, elements of the regiment carried out the My Lai massacre.

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.