[1][2] In the immediate emergency following Pearl Harbor, the United States had hurriedly sent a task force to defend New Caledonia against a feared Japanese attack.

It combined participation in numerous battles and campaigns but was marred by the My Lai massacre, which was committed by a platoon of the division's subordinate 11th Infantry Brigade, led by Lieutenant William Calley.

The division suffered a tactical defeat in the early morning of 28 March 1971, when Vietcong sappers assaulted FSB Mary Ann.

New Caledonia, a likely target for the Japanese for its critical strategic position on the lines of communication with New Zealand and Australia and its nickel and chromium mines, was now under the control of the Free French with a poorly equipped and trained native force and a company of Australian commandos.

The French objected to the arrangement and, increasingly worried about Japanese invasion being attracted by the airfield, demanded additional American forces.

With pressing needs to build up defenses in Hawaii and Australia, Army planners decided to put together a force rather than commit an already organized division.

Transshipment of troops and equipment was completed in Melbourne and the seven transports departed on 7 March (Australian time) for New Caledonia as convoy ZK-7,[8] arriving six days later.

On 28 March 1942, a unit of Force A left an isolated coconut plantation called Euralia for Port Havanah with two 2nd lieutenants and 22 men on a small auxiliary sailboat with equipment and 10 days' rations to begin their 'guerrilla' action by gathering the native headhunters and plantation workers from the nearby islands as reinforcements.In contrast to several other US Army divisions in the Pacific War, soldiers in the Americal Division received extensive weapons training as well as company- and battalion-level exercises in jungle terrain while at New Caledonia.

Largely because of transport constraints, the Americal arrived piecemeal and was fed into combat alongside the battle-hardened and exhausted US 1st Marine Division, which it eventually relieved.

The regiment was the first U.S. Army unit to engage in offensive action during World War II as part of the Battle of Guadalcanal.

Between 24 and 27 October, elements of the regiment withstood repeated assaults from Japanese battalions and inflicted some two thousand enemy casualties; the 164th also supported and participated in Marine attacks.

A. Vandegrift, was so impressed by the soldiers' stand that he issued a unit commendation to the regiment for having demonstrated "an overwhelming superiority over the enemy."

In addition, General Vandegrift took the unusual step of awarding Lieutenant Colonel Robert Hall, commander of the 3rd Battalion, 164th, with the Navy Cross for his role in these battles.

They were given the task (alongside the 37th Infantry Division and a Marine defense battalion) of holding and extending the right half of a previously established perimeter.

Despite ample warning and thorough defensive preparations, the battle soon degenerated into a bitter, close-quarters infantry affair, with artillery restricted by the need to avoid friendly troops and tanks unable to reach the scene.

The division went on the offensive in March 1944, driving the Japanese east of Mananga River, 7–9 April 1944, and seizing numerous strategic hill bases during the remainder of the month.

On 8 January 1945, the division began movement to Leyte and Samar, to take part in cleaning out remaining Japanese forces on those islands, and to invade Biri, Capul, Ticao, and Burias.

[13] A helicopter crew from the division's 123rd Aviation Battalion, led by Hugh Thompson, Jr., attempted to intervene in the massacre and were later awarded the Soldier's Medal.

[15] Calley was charged, convicted and sentenced to life imprisonment and hard labor on 31 March 1971 for the murder of 22 Vietnamese civilians.

Donaldson was the highest-ranking officer to undergo court-martial during the war, but charges were eventually dropped due to lack of evidence.

These allegations were raised by helicopter pilots under his command, and senior military investigators alleged that senior commanders of the division including Donaldson had routinely "...bet in the morning how many people they could kill – old people, civilians, it didn't matter", but the investigation was closed due to a lack of evidence.

[16][17] On 28 March 1971, Vietcong sappers attacked Firebase Mary Ann, which was being transferred by the 1st Battalion, 46th Infantry Regiment to the ARVN, resulting in 33 US/ARVN killed.

[18] The shoulder sleeve insignia's four white stars on a blue field are symbolic of the Southern Cross under which the organization has served.

[18] On the distinctive unit insignia, the blue saltire (cross of St. Andrew) alludes to New Caledonia in the Southwest Pacific where the division was created and first activated on 27 May 1942.