Adam Clarke on Joshua 11:20

It was of the Lord to harden their hearts - They had sinned against all the light they had received, and God left them justly to the hardness, obstinacy, and pride of their own hearts; for as they chose to retain their idolatry, God was determined that they should be cut off. For as no city made peace with the Israelites but Gibeon and some others of the Hivites, Jos 11:19, it became therefore necessary to destroy them; for their refusal to make peace was the proof that they wilfully persisted in their idolatry.