What are some historical examples, if any, of large-scale (say > 100 square kilometers) "no-man's lands" between states in perpetual conflict? These states would be mutually hostile, firmly divided perhaps by ideology or religion, but would not be engaged in an active war. The land in question might see some use by the native population for hunting or similar purposes but is effectively devoid of any settlement, any preexisting ones having been abandoned or destroyed.
The best example would remain depopulated for at least several decades, and be established on otherwise perfectly habitable territory.