The division was formed by the organization and redesignation of several other units. The 23rd Marine Regiment began as infantry detached from the 3rd Marine Division in February 1943, the same month that an artillery battalion of the 12th Marines became the genesis of the 14th Marines and engineer elements of the 19th Marines formed the nucleus of the 20th Marines. In March the 24th Marine Regiment was organized, and then in May it was split in two to supply the men for the 25th Marines.

This war-time shuffling provided the major building blocks for a new division. The units were originally separated, however, with the 24th Marines and a variety of reinforcing units (engineer, artillery, medical, motor transport, special weapons, tanks, etc.) at Camp Pendleton in California. The rest of the units were at Camp Lejeune, North Carolina. This East Coast echelon moved to Pendleton by train and transit of the Panama Canal in July and August. When all the units were finally together, the 4th Marine Division was formally activated on 14 August 1943, with Major General Harry Schmidt (pictured right) in command.

After intensive training, it shipped out on 13 January 1944, sailing from San Diego for the Marshall Islands, Roi-Namur in the Kwajalein Atoll. Operation "Flintlock" was the first objective for the Division. Six hours after landing on 1 February 1944, Roi Island was declared secured, and Namur Island was secured just 24 hours and 15 minutes after the first troops landed.

The 4th Marine Division set three new records on its first operation:
[1] It became the first Division to go directly into combat from the States;
[2] It was the first to capture Japanese mandated territory in the Pacific;
[3] It secured its objective in a shorter time than any other important operation since the attack on Pearl Harbor.

The 4th Marine Division landed on Saipan 15 June 1944. The severity of this battle was indicated by the 2,000 casualties suffered in the first two days of battle. The Flag was raised on Saipan after 25 grueling and bitter days of combat. The Division sustained 5,981 casualties killed, wounded and missing. This represented 27.6 percent of the 4th Marine Division's strength. The Japanese count was 23,811 known dead and 1,810 prisoners were taken.

The first wave of Marines to hit the Saipan beach in the Marianas invasion take cover behind a sand dune, while waiting for the following three waves to come in.
The 4th Marine Division landed on Tinian 24 July 1944. The island was defended by 9000 plus Japanese troops. This battle lasted nine days. The land assault on Tinian had cost the Division 290 men killed, 1,515 woulded and 24 missing in action. Approximately 9,000 Japanese troops were dead and 250 prisoners taken. In recognition of its work on Saipan and Tinian, the Fourth Division was awarded the Presidential Unit Citation.

Iwo Jima was the next destination for the 4th Marine Division which was now commanded by Major General Clifton B. Cates
(pictured left) .

The Japanese troops on Iwo Jima (inset right) numbered 23,000. The first wave of Marines hit the beach at 0902 on 19 February 1945. By the end of the second day casualties totaled 2,011. At 1800 on 16 March, 26 days and 9 hours after the first troops landed, Iwo Jima was declared secured. The greatest battle in Marine Corps history was over. The Division paid a heavy price, casualties numbered 9,098, which was almost half the division strength. An estimated 22,000 Japanese had been killed by the three Divisions (Third, Fourth, and Fifth), 8,982 having been counted in the Fourth's zone with only 44 prisoners taken by the Divison.

The combat record of the 4th Marine Division in World War II was exemplary. In the short space of one year and 19 days it participated in four major amphibious assaults and was awarded two Presidential Unit Citations and a Navy Unit Commendation. The Division suffered a total of 17,722 battle casualties during the four assault landings on Roi-Namur, Saipan, Tinian and Iwo Jima. (14,424 wounded and 3,298 killed or died of wounds.) and was deactivated 28 November 1945