The 10th Mountain Division (Light Infantry) is a light infantry division in the United States Army based at Fort Drum, New York. Formerly designated as a mountain warfare unit, the division was the only one of its size in the U.S. military to receive specialized training for fighting in mountainous conditions. More recently, the 10th Mountain has advised and assisted Iraqi Security Forces in Iraq and People's Defense Units in Syria.

Originally activated as the 10th Light Division (Alpine) in 1943, the division was redesignated the 10th Mountain Division in 1944 and fought in the mountains of Italy in some of the roughest terrain in World War II. On 5 May 1945, the division reached Nauders, Austria, just beyond the Reschen Pass, where it made contact with German forces being pushed south by the U.S. Seventh Army. A status quo was maintained until the enemy headquarters involved had completed their surrender to the Seventh. On 6 May, 10th Mountain troops met the 44th Infantry Division of Seventh Army.

Following the war, the division was deactivated, only to be reactivated and redesignated as the 10th Infantry Division in 1948. The division first acted as a training division and, in 1954, was converted to a full combat division and, in 1955, was sent to Germany before being deactivated again in 1958.

Reactivated again in 1985, the division was designated the 10th Mountain Division (Light Infantry) to historically tie it to the World War II division and to also better describe its modern disposition. Since its reactivation, the division or elements of the division have deployed numerous times. The division has participated in Operation Desert Storm (Saudi Arabia), Hurricane Andrew disaster relief (Homestead, Florida), Operation Restore Hope and Operation Continue Hope (Somalia), Operation Uphold Democracy (Haiti), Operation Joint Forge (Bosnia and Herzegovina), Operation Joint Guardian (Kosovo), and several deployments as part of the Multinational Force and Observers (Sinai Peninsula).

Since 2002, the 10th Mountain Division has been the most deployed regular Army unit. Its combat brigades have seen over 20 deployments, to both Iraq and Afghanistan, in support of Operation Iraqi Freedom and Operation Enduring Freedom.

In November 1939, two months after World War II broke out in Europe, during the Soviet Union's invasion of Finland, Red Army efforts were frustrated following the destruction of two armored divisions by Finnish soldiers on skis. The conflict caught global attention as the outnumbered and outgunned Finnish soldiers were able to use the difficult local terrain to their advantage, severely hampering the Soviet attacks and embarrassing their military. Upon seeing the effectiveness of these troops, Charles Minot "Minnie" Dole, the president of the National Ski Patrol, began to lobby the War Department of the need for a similar unit of troops in the United States Army, trained for fighting in winter and mountain warfare. In September 1940, Dole was able to present his case to General George C. Marshall, the Army Chief of Staff, who agreed with Dole's assessment, deciding to create a "Mountain" unit for fighting in harsh terrain. The U.S. Army authorized the formation of the platoon-sized Army Ski Patrol in November 1940. The first Patrol was formed at Camp Murray as part of the 41st Infantry Division under Lt. Ralph S. Phelps (later to become commanding General of the 41st). The army, prompted by fears that its standing force would not perform well in the event of a winter attack on the Northeastern coast, as well as knowledge that the German Army already had three mountain warfare divisions known as Gebirgsjäger, approved the concept for a division. This required an overhaul of U.S. military doctrine, as the concept of winter warfare had not been tested in the army since 1914. At first, planners envisioned ten mountain divisions, but personnel shortages revised the goal to three. Eventually, the 10th Mountain Division would be the only one brought to active duty. Military leaders continued to express concern about the feasibility of a division-sized mountain warfare unit until the fall of 1941, when they received reports that Greek mountain troops had held back superior numbers of unprepared Italian troops in the Albanian mountains during the Greco-Italian War. The Italian military had lost a disastrous 25,000 men in the campaign because of their lack of preparedness to fight in the mountains. On 22 October 1941, General Marshall decided to form the first battalion of mountain warfare troops for a new mountain division. The Ski Patrol would assist in its training.

On 8 December 1941, the day after the Japanese attack on Pearl Harbor and the subsequent American entry into World War II, the army activated its first mountain unit, the 87th Mountain Infantry Battalion (which was later expanded to the 87th Infantry Regiment) at Fort Lewis, Washington, south of Tacoma. It was the first mountain warfare unit in U.S. military history. The National Ski Patrol took on the unique role of recruiting for the 87th Infantry Regiment and later the division, becoming the only civilian recruiting agency in military history. Army planners favored recruiting experienced skiers for the unit instead of trying to train standing troops in mountain warfare, so Dole recruited from schools, universities, and ski clubs for the unit. The 87th trained in harsh conditions, including Mount Rainier's 14,411-foot (4,392 m) peak, throughout 1942 as more recruits were brought in to form the division. Initial training was conducted by Olympian Rolf Monsen. A new garrison was built for the division in central Colorado at Camp Hale, at an elevation of 9,200 feet (2,800 m) above sea level.

The 10th Light Division (Alpine) was constituted on 10 July 1943 and activated five days later at Camp Hale under the command of Brigadier General Lloyd E. Jones, with Brigadier General Frank L. Culin Jr. assigned as his assistant division commander (ADC). The 10th Light Division was centered on regimental commands; the 85th, 86th, and 87th Infantry Regiments. At the time, the division had a strength of 8,500 out of the 16,000 planned, so the military transferred troops from the 30th, 31st, and 33rd Infantry Divisions along with volunteers from the National Guards of Maine, New Hampshire, Vermont, New York, Michigan, Wisconsin, Minnesota, Iowa, North and South Dakota, Colorado, Wyoming, Montana, Idaho, Utah and Washington (specifically, men who were from the Rocky Mountain and northern states, close to the 45th parallel north), to fill out the remainder of the division. This lowered morale, and the division faced many difficulties in the new training, which had no established army doctrine. Also assigned to the division were the 604th, 605th, and 616th Field Artillery Battalions, the 110th Signal Company, the 710th Ordnance Company, the 10th Quartermaster Company, the 10th Reconnaissance Troop, the 126th Engineer Battalion, the 10th Medical Battalion, and the 10th Counterintelligence Corps Detachment. The 10th Light Division was unique in that it was the only division in the army with three field artillery battalions instead of four. It was equipped with vehicles specialized in snow operation, such as the M29 Weasel, and winter weather gear, such as white camouflage and skis specifically designed for the division.