In 1965, he deployed to Vietnam as a major. He served as an operations officer and battalion commander in the 101st Airborne Division. In November 1965, he founded the platoon-sized unit Tiger Force to "outguerrilla the guerrillas". Initially, Tiger Force was a highly decorated small unit in Vietnam which suffered heavy casualties and was awarded the Presidential Unit Citation. However, after Hackworth was promoted out of Vietnam, the unit began a string of atrocities and war crimes, with U.S. Army investigative records and interviews by "The Toledo Blade" estimating the unit eventually killed hundreds of noncombatants. Hackworth has stated he did not know about the atrocities and does not know what caused the unit to spiral out of control.