The 25^th Infantry Division was activated at Schofield Barracks, Territory of Hawaii, on October 1, 1941, at the same time the 24^th Infantry Division (now stationed at Fort Stewart, Georgia) was activated. Both divisions were comprised from elements of the famous old Hawaiian Division, in addition to two Hawaiian National Guard (HNG) Regiments. While the 24^th Division integrated the 19^th and 21^st Infantry Regiments, as well as the 299^th Infantry Hawaii National Guard, the 25^th Infantry Division was composed of the 27^th and 35^th Infantry Regiments, and the 298^th Infantry Hawaii National Guard. Other combat and support elements were added from the Hawaiian Division, and newly created or transferred units. The two new units operated for only ten weeks in peace before Japanese planes struck Oahu on the morning of December 7, 1941.

Evidence as to what extent Schofield Barracks was hit or Damaged is scant and unclear during the bombing and strafing by enemy aircraft of Wheeler Army Airfield. In the Division Staff Duty Journal, with one exception, no mention is made of Schofield Barracks being fired upon, although there are several reports about Wheeler being hit. Item 11 of the Journal recorded that at 0904 hours, one bomb hit the barracks of the 65^th Engineers, then in B Quad, "penetrating diagonally the roof to the 1^st floor, no personnel casualties". In fact, the "bomb" was a U.S. Navy 5-inch round fired, most likely, from Pearl Harbor, and ending up as a dud in a barrel of flour in "C" Company’s kitchen. Schofield Barracks did not enter into the planing for the attack on Pearl Harbor. As Japanese planes bombed and strafed Wheeler, some of the enemy may have made passes at Schofield. To clarify another point: the striking aircraft did not fly through Kole Kole Pass, although it is possible that in the ensuing confusion it might have seemed that way.

Immediately following the attacks on Wheeler as well as on other airfields on Oahu and Pearl Harbor, both the 24^th and 25^th Infantry Divisions were dispatched to their defensive positions: the 24^th to the North Shore, and the 25^th to the beaches on the south-side of the Island. The following year was spent in these defensive positions, as well as taken up by intensive jungle training.

On November 25, 1942, the 25^th Division began its deployment by troop ships to the South Pacific, landing on Guadalcanal between December 17, 1942 and January 4, 1943. The Division received orders almost immediately to launch an attack against the strong Japanese forces, which other Army and Marine contingents had been fighting doggedly for nearly five months. The Division jumped off on the morning of January 10, 1943. After a month of bitter combat, it succeeded in defeating the enemy forces. The 25^th Infantry Division had completed its first wartime mission with speed and aggression. The next action of the Division took place in the Solomons by clearing the Arundel and Kolombangera Islands held by the enemy, and participating in the capture of Vella LaVella.

Following the Solomon campaigns, the Division headed for New Zealand, and subsequently New Caledonia, for a period of rest as well as for intensive training. On January 11, 1945, the Division landed at Luzon in the Philippine Islands. It quickly drove from Lingayen Gulf to the main highway through Balete Pass, joining forces with the 32^nd Infantry Division. The 25^th established a record of 165 consecutive days of combat. For its service in Luzon, the 25^th Infantry Division was awarded the Philippine Presidential Unit Citation, as well as six other units citations. In the wake of the Philippines campaign, the 25^th Infantry Division began preparing for the invasion of Japan. However, World War II had ended before an invasion was actually launched and the "Tropic Lightning" Division was sent to Osaka, Japan, as part of the Army of Occupation. Six Medals of Honor were awarded to Division soldiers during WW II.

Open warfare again flared in Asia the 25th of June 1950. The North Korean People’s Army crossed the 38th Parallel in an unprovoked attack on the Republic of South Korea. Acting under United Nation’s orders, the "Tropic Lightning" moved to Korea between 5 and 18 July 1950.

The Division under the command of Major General William B. Kean, successfully completed its first mission, the blocking of approaches to the port city Pusan. For this action, the "Tropic Lighting" received its first Republic of Korea Presidential Unit Citation.

A break-out of the Pusan perimeter was followed by a successful drive into North Korea in October 1950. On 24 November, the Division joined "Task Force Dolvin" and successfully drove the enemy to the Yalu River. In a sudden reversal, an overwhelming number of Chinese Communist troops crossed the Yalu and pushed back United Nations forces all along the front. The Division carried out an orderly withdrawal and it was ordered to take up defensive positions on the south bank of the Chongchon River, on 30 November 1950. These lines failed, but after short withdrawals, a permanent battle line was established south of Osan.

A new offensive was launched on 15 January 1951, and was successfully completed on 10 February with the capture in Inchon and Kimpo Air Base. The Division next participated in Operation "Ripper", driving the enemy across the Han River. Continued success came with Operation "Dauntless", "Detanote", and "Piledriver" in the spring of 1951. These offensives secured the famous "Iron Triangle" for the UN. When UN and North Korean negotiators began their meetings in the summer of 1951, the Division activity slowed to patrol and defensive actions, maintaining the present Main Line of Resistance. This type of action continued into the winter of 1952.

On 5 May 1953, the Division assumed the responsibility of guarding the approaches of Seoul. A heavy assault was hurled at it during 28-30 May 1953. The assault was repulsed. In July, the Division again moved to reserve status at Camp Casey. On 27 July 1953, the Armistice was signed.

For its action from 5 May 1953, to July 9, 1953, the successful defense of Seoul area, the Division received its second Republic of Korea Presidential Unit Citation. Fourteen "Tropic Lightning" soldiers were awarded Medals of Honor. The Division remained in Korea until 1954, and returned to Hawaii during September and October of that year. After twelve years of absence, the 25th Infantry "Tropic Lightning" Division had come home.

After its return to Hawaii in 1954, the Division established and conducted intensive training programs in jungle warfare techniques and the study of Asian languages, thus becoming the only trained counter-guerilla unit in the United State Army. In response to a request from the U.S. Military Assistance Command in Vietnam, the Division sent 100 helicopter door-gunners to the Republic of South Vietnam in early 1963. By the time this phase was completed in the latter part of 1965, 2200 men of the 25th Infantry "Tropic Lightning" Division had been involved.

In August of 1965, Company "C", 65th Engineer Battalion was deployed to South Vietnam to assist in the construction of port facilities at Cam Ranh Bay. In December of that year, responding to a MAC-V request for additional combat support, the Division sent the 4000 men 3rd Brigade. "Operation Blue Light" was the largest and longest airlift of personnel and cargo into a combat zone in military history. The Brigade deployed from Hickam Airforce Base, Honolulu, to the central highlands at Pleiku, the first soldiers arriving on December 24, 1965. By mid-January, the operation was completed. The balance of the Division closed by Spring, 1966.

From April 1966 until 1969, the Division was heavily engaged throughout its area of operations. During the Tet offensives of 1968 and 1969, "Tropic Lightning" personnel were instrumental in defending the besieged city of Saigon. Thereafter, from May of 1969 through 1970, the 25th Infantry Division became involved in the Vietnamization Program rather than in actual combat. However, during the period April through June 1970, "Tropic Lightning" participated in Allied thrusts deep into enemy sanctuaries located in Cambodia. In these operations, it confiscated thousands of tons of supplies, and hundreds of weapons.

Following its return from Cambodia to South Vietnam, the Division resumed its place in the Vietnamization Program. By late December 1970, elements of the 25th Infantry Division were able to begin redeployment to Schofield Barracks. The second Brigade was the last element of the "Tropic Lightning" Division to depart Vietnam and closed on Schofield Barracks in the early days of May 1971. During the war in Vietnam, twenty-two Medals of Honor were awarded to "Tropic Lightning" soldiers.

After its return to Schofield Barracks, the 25th Infantry Division remained the only Army Division never to have been on the mainland. It was reduced to a single brigade, numbering about 4000. The Division was reactivated in March of 1972, when it was reorganized. Early in 1973, the 29th Brigade of the Hawaii National Guard, and the 100th-442nd Battalion of the Army Reserves, were designated as the 25th Infantry Division’s roundout-brigade, to be ready in the event of a national emergency.

As the Pacific Command Reserve, the 25th Infantry Division today trains throughout the Pacific theater. The Division participates in exercises in diverse location, such as Japan, Australia, Thailand, Korea and other Pacific basin locations. Local training and observation programs have been established with other nations in the region too.

In 1985, the Division began its reorganization from a conventional Infantry Division to a Light Infantry Division. The primary characteristics of the new Light Infantry Division are:

a) mission flexibility;

b) rapid deployment;

c) combat readiness at 100% strength;

d) Pacific orientation.

Major configurative changes are the addition of a third Brigade, one additional direct-support Artillery Battalion, and the expansion of the Combat Aviation Battalion to a Brigade. The over-all authorized strength of the Division, goes from 13,025 to 10,700 and due to the reduction of large quantities of heavy equipment, the 25th Infantry Division is "light". The reorganization was completed on 1 October 1986.

One platoon each from Aco, Bco, and Cco, 4th Battalion, 27th Infantry, deployed to Saudi Arabia on January 30, 1991, scheduled to be replacement squads as needed in the ground campaign. But after observing their thoroughly outstanding performance in desert warfare training, the Assistance Commander of Third U.S. Army asked for them as security forces for Army Forward Headquarters.

They attacked with Third Army Forward into Kuwait City on February 26, secured the headquarters area, and conducted "hairy" mop-up operations in the city and adjacent fields. On March 1, Company A’s platoon accompanied General H. Norman Schwarzkopf into Iraq to provide security at the truce signing. The three platoons returned to Schofield Barracks, without casualties, on March 20, 1991.

The Division has sent troops to Haiti replacing soldiers of the 10th Mountain Division in 1994. Also, the Division continues to deploy troops to Egypt to participate in the MFO. Other major scheduled exercises are "Cobra Gold" in Thailand, "Kangaroo" in Australia and "Orient Shield" in Japan, ("Team Spirit" in Korea, which has been put on hold).