Arnum was formerly part of the German Empire. In the German Empire, the place was called Arnum.
The place is now called Arnum and belongs to Denmark.
| Historical place name | Country | Administration | Time |
|---|---|---|---|
| Arnum | German Empire | Hadersleben | before the Versailles Treaty |
| Arnum | Denmark | Tønder | after the Versailles Treaty |
| Arnum | Denmark | Sønderjylland | 1993 |