Indiantown Gap derives its name from the various Native American communities that resided in this region of Pennsylvania. The first inhabitants were Susquehannocks, an Iroquois tribe first encountered by Europeans in the 17th century. In the mid-1700s, Scotch-Irish, English and German pioneers settled the region and managed to live peacefully with the neighboring Lenape Indians. During the French and Indian War, however, tribes who were allied with French colonists raided many English frontier settlements. As Indiantown Gap increasingly became the site of frequent battles, pioneers built a number of defensive structures, including Swatara Fort, Harpers Fort and Reeds Fort.
In the 1930s, when the Pennsylvania National Guard needed a larger area for training maneuvers and firing ranges, the government authorized the acquisition of 12,047 acres in Dauphin and Lebanon counties. The 55th Infantry Brigade was the first unit to use Fort Indiantown Gap when it held its annual maneuvers at the reservation in summer 1932. The following year, the 53rd Field Artillery first trained at Indiantown Gap, and in 1934, the 28th Infantry Division and 52nd Cavalry Brigade were assembled there. Over 100 buildings from nearby Mount Gretna—including officers' mess halls, administration buildings, latrines and bathhouses—were dismantled and hauled by truck to the present location at Indiantown Gap.
After World War II, Indiantown Gap became a separation center for officers and enlisted men returning from overseas, and eventually home to the 32,000 troops of the 5th Infantry Division and a training center during the Korean War. From 1962 to 1973, Indiantown Gap was the host installation for the largest Reserve Officers Training Corps advanced summer camp nationwide. During this 11-year period, 41,158 cadets completed training. In 1975, Fort Indiantown Gap became a camp for Southeast Asian refugees. For eight months, more than 22,000 Vietnamese and Cambodian refugees were resettled through this facility.
In 1976, a section of Fort Indiantown Gap was selected as the national cemetery for the states of Delaware, Maryland, New Jersey, Virginia and West Virginia. The Commonwealth of Pennsylvania donated Land for the 677-acre site to the Veterans Administration.