During World War II, Fort Missoula was turned over to the Department of Justice, Immigration and Naturalization Service, for use as an Alien Detention Center. Between 1941 and 1944, the ADC held 1,200 non-military Italian men, 1,000 Japanese resident aliens, 23 German resident aliens, and 123 Japanese Latin and South Americans.
Fort Missoula's ADC was established to hold foreign nationals and resident aliens, to distinguish it from the 10 better known War Relocation Act camps that held 120,000 Japanese Americans.
The 1,200 Italian men were merchant seamen, World's Fair employees and the crew of an Italian luxury liner seized in the Panama Canal. Many of the Italians, who referred to the Fort as "Bella Vista," spent the war as paid laborers replacing American men working in forestry, farms, the sugar beet industry and constructing Highway 12.
Following the bombing of Pearl Harbor on December 7, 1941, the FBI arrested more than a 1,000 of the most prominent Japanese leaders on the west coast as potential security risks. Ultimately, over 1,000 Japanese men-all resident aliens barred by law from American citizenship-were held at Fort Missoula for loyalty hearings. None was ever charged with any act of disloyalty but all were held at Fort Missoula or other camps for the duration of the war. The courtroom in Post Headquarters where Enemy Alien Hearings were held is being restored by the Historical Museum with funding from the National Park Service.
A handful of German resident aliens were held for short periods at Fort Missoula, although most were held at Fort Lincoln in Bismarck, North Dakota. The 123 men of Japanese ancestry from Latin and South America, mostly Peru, were a very small part of several thousand held primarily at the Santa Fe camp or at Crystal City, Texas.
Fort Missoula is currently the largest intact WWII internment site with most major buildings of the era still in use, including the Post Headquarters with its courtroom, the hospital, commissary, officer and staff housing, barracks and other support structures. The Historical Museum has an exhibit on internment housed in a restored barrack and is restoring the Post Headquarters.