Charles Ellicott Commentary Judges 11:21

"And Jehovah, the God of Israel, delivered Sihon and all his people into the hand of Israel, and they smote them: so Israel possessed all the land of the Amorites, the inhabitants of that country." — Judges 11:21 (ASV)

The Lord God of Israel. — This is evidently a cardinal point in the mind of Jephthah. The God of Israel has decided against the gods of Ammon.

All the land of the Amorites. — All the land, therefore, which they took from the Amorites was theirs by the immemorial law of nations, irrespective of any who had been its previous owners (Grot., De Jure Belli, 3:6, § 7).