The 5th Marine Division was a United States Marine Corps infantry division which was activated on 11 November 1943 (officially activated on 21 January 1944) at Camp Pendleton, California during World War II. The 5th Division saw its first combat action during the Battle of Iwo Jima in 1945 where it sustained the highest number of casualties of the three Marine divisions of the V Amphibious Corps (invasion force). The 5th Division was to be part of the planned invasion of the Japan homeland before Japan surrendered. Assault troops of the 5th Division were included in the Presidential Unit Citation awarded to the V Amphibious Corps for extraordinary heroism on Iwo Jima from 19 to 28 February 1945. The 5th Division was deactivated on 5 February 1946.
The 5th Division was ordered to be reactivated on 1 March 1966 at Camp Pendleton, California, during the Vietnam War. The division, beginning with the reactivation of Regimental Landing Team 26 (RLT 26), was expected to be fully manned within one year; the 5th Division was never in command of the 26th Marine Regiment (26th Marines) in the war. In December, all three infantry battalions of the 26th Marines were fighting in Vietnam attached to the 3rd Marine Division. By June 1967, the 5th Division was ready to deploy anywhere. It was never intended that the 5th Division would go overseas. It was a force in readiness. But in February 1968, General William C. Westmoreland, U.S. Army, commander of U.S. forces in South Vietnam, asked for help because of the all-out Communist Tet Offensive. The 27th Marine Regiment (27th Marines), 5th Marine Division, was airlifted out on 48 hours notice, with 3,700 Marines. In September, it became the first major combat unit to come home from the Vietnam War. The 5th Marine Division formally deactivated on 26 November 1969.
