Jephthah introduces theology into the negotiation for the first time. Yet he is not saying anything particularly novel here; it is simply part of the tradition he has been using all along, namely, from Numbers 21:21–34 and Deuteronomy 2:24–36. That said, in the thinking of the day, the outcome of battles was in general understood to be divinely determined, and so Jephthah’s reference to Yahweh would have probably made much sense to Jephthah’s opponent, and could have been expected to carry some weight.1
21 And the LORD, the God of Israel, gave Sihon and all his people into the hand of Israel, and they defeated them. So Israel took possession of all the land of the Amorites, who inhabited that country.