“And they possessed all the border of the Amorites, from Arnon even to Jabbok, and from the wilderness even to Jordan. So now Yahweh, the God of Israel, has dispossessed the Amorites from before his people Israel, and should you possess them?”
So Jephthah emphasised that their right to possession of the land was because they had possessed it when Yahweh had dispossessed the Amorites on their behalf. Thus the Ammonites and Moabites had no right of possession such as they claimed. Let them beware. Yahweh would not be pleased with their claims.
Note what he was trying to do. He was not denying that the Ammonites could argue that if they conquered it then it meant that their god had given them the land as against Yahweh. He would have accepted that as being correct. But what he wanted them (and Yahweh and Chemosh and Melek) to recognise was that if they did so it was by right of conquest, not because of any previous rights. They had no justification other than conquest. Thus no nationalistic pride was involved. They had no inherent right to it.