The 104th Infantry Division was first constituted on 24 June 1921 as the 104th Division, before being organized and activated in October of that year in Salt Lake City, Utah.  Assigned to the division were the 207th and 208th Infantry Brigades, containing the 413th, 414th, 415th, and 416th Infantry Regiments. As a unit of the Organized Reserves, the division represented assets from the states of Montana, Wyoming, Idaho, Utah, and Nevada. In 1924 it received its shoulder sleeve insignia. The division would not see significant duty until World War II.

World War II

At the outbreak of World War II, the 104th Division began preparing to participate in the war in Europe. The division was ordered into active military service on 15 September 1942 and reorganized as the 104th Infantry Division at Camp Adair, Oregon. The 207th and 208th Brigades did not reactivate as part of an army-wide elimination of brigade commands within its divisions. The division was instead centered around three infantry regiments; the 413th Infantry Regiment, the 414th Infantry Regiment, and the 415th Infantry Regiment. Also assigned to the division were the 385th, 386th, 387th and 929th Field Artillery Battalions, as well as the 104th Signal Company, the 804th Ordnance Company, the 104th Quartermaster Company, the 104th Reconnaissance Troop, the 329th Engineer Battalion, the 329th Medical Battalion, and the 104th Counter Intelligence Detachment. From that point it began training as a division in preparation for deployment to Europe. The division trained in the northwestern United States during the next two years, earning its name "Timberwolf Division" from its time in the area. The division was the first Army Division to train specifically for fighting in nighttime conditions. After training at Camp Adair, the division participation in the Oregon Maneuver combat exercise in the fall of 1943.

Europe

The 104th Infantry Division sailed for Europe on 27 August 1944. It landed in France on 7 September 1944. In early October, Major General Terry de la Mesa Allen, Sr. took command of the division. He would command the division during most of its time in combat. The division was assigned to III Corps of the Ninth United States Army, Twelfth United States Army Group. The division then organized and assembled at Manche, France before heading into combat.

Joining the Battle of Hürtgen Forest, the division moved into defensive positions in the vicinity of Wuestwezel, Belgium on 23 October 1944. Three days later, it went on the offensive, taking Zundert, gaining control of the Breda-Roosendaal Road and overrunning Vaart Canal defenses. Leur and Etten fell as the division advanced to the Mark River, arriving there by 31 October. A coordinated attack over the Mark River at Standaarduiten on 2 November established a bridgehead and the rest of the division crossed the river. Zevenbergen was captured and the Maas River was reached on 5 November. While the bulk of the division moved near Aachen, Germany, elements remained to secure Moerdijk until 7 November, when they were relieved. During this time, the division was reassigned to VII Corps of the First United States Army, also part of the Twelfth Army Group.

On 16 November 1944, the division went on another offensive, taking Stolberg and pushing on against heavy resistance. Eschweiler fell on 21 November and the enemy was cleared from the area west of the Inde River, including Inden by 2 December 1944. Lucherberg was held against enemy counterattacks on 3 December, and all strongholds west of the Roer River were captured by the 23rd. It took temporary command of the 60th Infantry Regiment of the 9th Infantry Division. During the Battle of the Bulge, the 104th actively defended its sector near Duren and Merken from 15 December to 22 February 1945. During that time, it was reassigned to XIX Corps of the Ninth United States Army. It then moved across the Roer taking Huchem-Stammeln, Birkesdorf, and North Duren. On 5 March, after heavy fighting, it entered Köln. After defending the west bank of the Rhine River, the division crossed the river at Honnef on 22 March 1945, and attacked to the east of the Remagen bridgehead. During this time, some of the division's assets fell under command of the 1st Infantry Division and the 3rd Armored Division. After a period of mopping up and consolidation, it participated in the trap of enemy troops in the Ruhr pocket. The 104th repulsed heavy attacks near Medebach and captured Paderborn on 1 April 1945. After regrouping, it advanced to the east and crossed the Weser River on 8 April, blocking enemy exits from the Harz Mountains. The division then crossed the Saale River and took Halle in a bitter five-day struggle from 15 to 19 April. The sector to the Mulde River was cleared by 21 April, and after vigorous patrolling, this Division contacted the Red Army at Pretzsch on 26 April. The division took temporary command of assets from the 69th Infantry Division in early May.

Demobilization

The division returned to the United States on 3 July 1945. Once back in states, it continued the process of demobilization until 20 December of that year, when it was inactivated. The division suffered 1,294 killed in action, 5,305 wounded in action, 385 missing in action, and 27 prisoners of war. The division suffered a further 6,396 non-battle casualties, for a total of 13,407 casualties. The division took 51,727 German prisoners during the war, most of whom surrendered following the armistice.

During World War II, soldiers of the division were awarded two Medals of Honor, 14 Distinguished Service Crosses, one Distinguished Service Medal, 642 Silver Star Medals, six Legion of Merit medals, 20 Soldier's Medals, 2,797 Bronze Star Medals, and 40 Air Medals. The division received 9 Distinguished Unit Citations and three campaign streamers during 200 days of combat.

Training Division

The division was reactivated on 1 December 1946 in the organized reserves in Portland, Oregon. It began taking on the responsibility of holding training programs for new soldiers of the US Army Reserve. In July 1948, the division held its first session of summer training. By the end of the training, it had turned out 300 new reservists. By 1952, the division was turning out 1,500 new reservists per training camp. The division was reorganized specifically as a training division in 1959. In 1961, the division was relocated to Vancouver Barracks, Washington.

In 1967, the division was reorganized. As part of an army wide initiative known as the Reorganization Objective Army Division plan, the division's regiments were disbanded and replaced with larger and more versatile brigades. The 1st Brigade, 104th Division, activated at Vancouver Barracks, and the 2nd Brigade, 104th Division activated at Pasco, Washington. Meanwhile, the 3rd Brigade, 104th Division, as well as the 4th Brigade, 104th Division both activated at Fort Lawton, Washington. Each of these brigades carried the history of other historic units which fought under the 104th Infantry Division in World War II. The 104th Division was then assigned the mission of conducting One Station Unit Training, Basic Combat Training, Advanced Individual Training, and Combat Support training. 1st Brigade took on basic combat training, while 3rd Brigade undertook combat support training, 4th Brigade conducted combat service support training.

In 1996, three more brigades were added to the division's structure. The 5th Brigade, 104th Division was activated at Salt Lake City, Utah. The 6th Brigade, 104th Division was activated at Aurora, Colorado. The 7th Brigade, 104th Division activated at Vancouver, Washington. The 5th Brigade conducted health services training, 6th Brigade took charge of professional development training and 7th Brigade provided training support to the other brigades. These units were re-designated from other training commands and put under the command of the division.

Two additional provisional brigades were created under the 104th Division in 1999; the 8th Brigade, 104th Division was created at Fort Lewis as a unit for training Reserve Officers' Training Corps cadets, and the 4690th US Army Reserve Forces School at Fort Shafter, Hawaii was re-designated as the 4690th Brigade, 104th Division, for service as a multifunctional training unit. In 2005, the Base Realignment and Closure suggestions included the closure of the Vancouver Barracks, and the 104th Division was subsequently relocated to Fort Lewis, Washington. In 2005, the Distinctive Unit Insignia was designed by Captain Roderic W. Langer under the direction of Major General Terrill K. Moffatt. The 104th received its new distinctive unit insignia in 2006.

In October 2007, the division was renamed the 104th Division (Leader Training). This change reflected a change in the division's mission, specifically training officer and non-commissioned officer candidates in their assigned fields.