@Simkin : There's also the option of the land having been under demon's control/rule before but then human took it over in their expansions and refused to recognize the demon's rights over the land, then the war just goes on because even when the demon win and stop expanding the human eventually come back to 'reclaim' 'their' land.
Even more so if the demons include races that live with nature so to the humans the place don't appear occupied.