“ Will you not possess that which Chemosh your god gives you to possess? So whoever Yahweh our God has dispossessed from before us, them we will possess.”
Chemosh was in fact the god of Moab, not the god of Ammon. Their god was Melek (Molech, Milcom). Thus many have claimed that Jephthah here made a mistake. But he has made no mistake. The king of Ammon was arguing about and laying claim to land that had in times past, before the Amorites had captured it, belonged to Moab, and he was making his claim on those very grounds (Judges 11:13). From his viewpoint that land had once belonged to Chemosh. So Jephthah wanted him to face up to the fact that it was Chemosh who had relinquished it to the Amorites (Numbers 21:29).
Essentially, he was saying, it was Chemosh, their own god (one of the gods of the confederacy) who had not given its possession to the Moabites, nor to the Ammonites, and it was this Chemosh to whom the king of the Ammonites was in the last resort appealing, Chemosh who had given it to the Amorites. Let them therefore possess what he had patently given to them, and recognise that he gave that other land to the Amorites and that Yahweh has take that land from the Amorites and given it to Israel. And that that is why they now claimed possession of it.
Once we recognise that the king of Ammon was speaking on behalf of an Ammonite/Moabite alliance (which he had to be to make the claim for the land that he made) the difficulty disappears. He was speaking on behalf of both Melek and Chemosh, and in relation to that particular land, of Chemosh. It was Chemosh who could theoretically claim a past right to the land, not Melek.
We must also recognise the possibility that Jephthah was cleverly trying to sow the seeds of division between the two allies. If he could get them to argue Melek against Chemosh, and that it was the king of Moab who should be asking for the land and not the king of Ammon, he would have divided their ranks.
At this point we can consider the effect these arguments, read out before his own men, were having on them. They would be chuckling and cheering and feeling strongly fortified. And his hope was that when the Ammon/Moabite leadership and their men heard it they would be feeling the opposite.
Jephthah now went on to point out that their delay in making this claim itself demonstrated that they had no case, and that no one in the past had dared to argue with Israel about it.