Adam Clarke on Judges 11:23

The Lord God of Israel hath dispossessed the Amorites - Jephthah shows that the Israelites did not take the land of the Moabites or Ammonites, but that of the Amorites, which they had conquered from Sihon their king, who had, without cause or provocation, attacked them; and although the Amorites had taken the lands in question from the Ammonites, yet the title by which Israel held them was good, because they took them not from the Ammonites, but conquered them from the Amorites. So now the Lord - hath dispossessed the Amorites. - The circumstances in which the Israelites were when they were attacked by the Amorites, plainly proved, that, unless Jehovah had helped them, they must have been overcome. God defeated the Amorites, and made a grant of their lands to the Israelites; and they had, in consequence, possessed them for three hundred years, Jdg 11:26.