Charles Ellicott Commentary Joshua 11:20

"For it was of Jehovah to harden their hearts, to come against Israel in battle, that he might utterly destroy them, that they might have no favor, but that he might destroy them, as Jehovah commanded Moses." — Joshua 11:20 (ASV)

It was of the Lord to harden their hearts ... that he might destroy them. —Or rather to strengthen their heart — i.e., render them obstinate. These words go to prove what has been said elsewhere, that the conquest of Canaan was not intended to be a massacre of the unresisting inhabitants.