The 5th Marine Division was a United States Marine Corps ground combat 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 South 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.

The 5th Marine Division was activated on Armistice Day, 11 November 1943. The division's Headquarters Battalion officially began operating at Marine Corps Base Camp Pendleton on 1 December, at which time men and equipment began streaming into Camp Pendleton. The official activation date for the Division was 21 January 1944.

The division was formed around a solid core of combat veterans and experienced cadremen; among the personnel were Marines from the former 1st Marine Parachute Regiment, the Raider Training Battalion, the Parachute Training School, West Coast, and the Parachute Replacement Company. However, there were many issues obtaining the total required number of Marines for the division, as the Marine Corps also had to provide combat replacements to other divisions as well as staff the newly-formed 6th Marine Division.

Parts of the division began to deploy overseas to act as the reserve force during the Battle of Guam where they were not needed. Because of this they were sent to Camp Tarawa near Hilo, Hawaii for further training. While there the 31st Naval Construction Battalion was attached to the Division tasked as shore party for the upcoming operation. After more extensive training the division loaded ships and left Hawaii in January 1945. By mid-February they were sailing past Saipan headed for Iwo Jima.

(Presidential Unit Citation: Assault Troops, Fifth Amphibious Corps, 19 to 28 February 1945)

The 5th Division landed on beaches red 1, red 2 and green 1 at the base of Mount Suribachi on 19 February 1945 on Iwo Jima. The division sustained heavy initial losses, so much so that by that afternoon, the 26th Marine Regiment (26th Marines) had to be released as the division reserve. On 23 February, two American flags were raised on Mount Suribachi by members of the 28th Marine Regiment (28th Marines). The 5th Division would fight on Iwo Jima from 19 February until 26 March where they would sustain 2,482 killed in action, 19 missing in action, and 6,218 wounded in action. This was the highest casualty rate among the three Marine divisions involved in the invasion. The 5th Division began loading onto ships on 26 March, finally leaving Iwo Jima on 27 March 1945 sailing for Hawaii.

On 21 March 1945 the 5th Marine Division Cemetery was formally dedicated on Iwo Jima. Chaplains prayed, Major General Keller E. Rockey added a tribute to the dead and Lieutenant Roland B. Gittelsohn, U.S. Navy and a 5th Division chaplain, spoke of friends buried and "the ghastly price of freedom...." The American flag (there since 14 March) at the northern part of Iwo Jima was raised then lowered to half-staff. Taps was played echoing across the dark foreboding ash of "Sulfur Island".