Judges 11:21

And the Lord God of Israel delivered Sihon and all his people
into the hand of Israel
So that as Sihon, his people, and his country, fell into the hands of Israel through the victory the Lord gave them over him, they had a divine right to the land now in dispute:

and they smote them:
destroyed him and all his people, as they were ordered to destroy the seven nations of Canaan, of which the Amorites were one, ( Deuteronomy 7:1 ) ,

so Israel possessed all the land of the Amorites, the inhabitants of
that country;
by means of the above victory they came into the lawful and rightful possession of all the land that belonged to the Amorites, who were at that time, and none else, the inhabitants of it; and therefore the Ammonites could have no claim to it, nor was any made till now.