Germany set up a special category of internment camps (Internierungslager, or Ilags) for civilians from enemy nations, including France and Britain, as well as the United States and other Allied countries. There were at least twelve of these camps (eight within the Reich and four in occupied Western Europe), which were operated by the Wehrmacht, although they didn't all exist at the same time, as some camps were closed and/or relocated during the war.

Since these camps were administered by the Wehrmacht, Ilags within the Reich were given Roman numeral designations based on the Defense District (Wehrkreis) in which they were located, following the schematic used for prisoner of war camps; camps outside the Reich were named after their locations. The known camps are: Ilag IV (Eisenberg; now Jezeří, Czechia), Ilag VII (Laufen, Germany), Ilag VII/Z (a branch camp, or Zweiglager, of Ilag VII, in Tittmoning, Germany), Ilag VIII (Tost; now Toszek, Poland), Ilag VIII/Z (Kreuzberg; now Kluczbork, Poland), Ilag XIII (Wülzburg, Germany), Ilag XVIII (Spittal an der Drau, Austria), Ilag Dongelberg (Belgium), Ilag Giromagny (France), Ilag St. Denis (France), and Ilag Vittel (France).

Conditions in the Ilags were generally good, usually significantly better than those experienced by Western Allied prisoners of war; however, cases of discrimination against Jewish internees were reported after the war, and some Polish Jews with South American passports were deported from Ilag Vittel. The treatment of the internees was inspected by international observers from the Red Cross. Neutral countries like Switzerland assumed a supervisory role over the internees similar to the function of the protecting powers in the treatment of prisoners of war (although this wasn't mandated by the Geneva Convention as it was in the case of POWs).

The Germans used structures like resort hotels and castles to accommodate the internees. They were generally allowed a relatively high degree of self-administration, and the Germans and international relief agencies provided them with enough supplies to maintain active religious, athletic, educational, and cultural programs within the camps. Most of the internees were held until the end of the war, although people who became ill were sometimes repatriated to their home countries for medical treatment. The remaining internees were liberated by Allied forces in 1944 and 1945.

The internees at Ilag Vittel had a unique and horrifying experience, as the Germans briefly used the Ilag as a transit camp for a transport of Jews and French résistants who were being deported to concentration camps in August 1944. The internees at Vittel provided food to these deportees while they were held there, before the transport moved on.

Sources:

Gianfranco Mattiello and Wolfgang Vogt, Deutsche Kriegsgefangenen- und Internierten-Einrichtungen 1939-1945. Handbuch und Katalog: Lagergeschichte und Lagerzensurstempel, vol. 2 (Koblenz, 1987), pp. 170-173

Also see The United States Holocaust Memorial Museum Encyclopedia of Camps and Ghettos, Vol. 4 (Indiana UP, 2022)

There are a few published memoirs from former internees about their experiences in the Ilags, mainly from British and French internees. Additional primary source documentation can be found in the relevant national archives, including the British National Archives, Kew and the National Archives and Records Administration, College Park, MD, and the Military Archive of the German Federal Archive, Freiburg.