How would Jephthah’s argument have been understood by Ammon?

Jephthah argues that Israel gets to possess whatever the Lord gives them, and so the Ammonites must be satisfied with whatever they have received from their god. Jephthah is using an argument that was common in the ancient Near East. That is, each nation had its own god whose job included providing his people with a homeland. Whatever land your god gave you belonged to you.

Some believe that Jephthah is deliberately accommodating himself to the Ammonites’ worldview, even though he knows this view is mistaken, since the Lord is the only God. But it is more likely that, given his personal history as a bandit in the land of Tob, he actually believes what he is saying. He has clearly adopted some pagan ideas about God. He likely believes that the Lord has a limited, localized domain, just like Chemosh. This error is only made more severe by the fact that Deuteronomy 2:19 specifically states that the Lord (not Chemosh) has given the Ammonites their own land as their possession. Jephthah’s thinking is both polytheistic (believing in multiple gods; see Judges 10:6, Judges 10:10, Judges 10:13–14 for this in Israel’s thinking) and syncretistic (mixing the true religion with false religion).¹ He should have taken the opportunity to declare that the Lord is the only true God, so Ammon should not dare to make war against him. Yet he has been shaped by a culture of idolatry.