See the marginal references.

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, Joshua 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.

For it was of the Lord to harden their hearts,.... As he hardened the hearts of Pharaoh and the Egyptians, that his power might be displayed in their destruction:

that they should come against Israel battle; and so fall in it:

that he might destroy them utterly; for their abominable wickedness, idolatry, incest, &c. they had been guilty of:

and that they might have no favour; which they would have had, had they made peace as the Gibeonites did; or that they might not pray and make supplication, the Lord not giving them a spirit of supplication, but an hard heart, as Gussetius (f) observes the words may be interpreted, though he seems to prefer the former, sense:

but that he might destroy them, as the Lord commanded Moses; Deuteronomy 7:1.

(f) Comment. Ebr. p. 272.

For it was of the LORD to {l} harden their hearts, that they should come against Israel in battle, that he might destroy them utterly, and that they might have no favour, but that he might destroy them, as the LORD commanded Moses.

(l) That is, to give them over to themselves: and therefore they could not but rebel against God and seek their own destruction.

11:20 To harden their hearts - It was the design of God's providence not to soften their hearts to a compliance with the Israelites, but to give them up to their own animosity, pride, confidence and stubbornness; that so their abominable and incorrigible wickedness might be punished, and that the Israelites might not be mixed with them, but be entire among themselves in the possession of the land.

11:15-23 Never let the sons of Anak be a terror to the Israel of God, for their day to fall will come. The land rested from war. It ended not in a peace with the Canaanites, that was forbidden, but in a peace from them. There is a rest, a rest from war, remaining for the people of God, into which they shall enter, when their warfare is accomplished. That which was now done, is compared with what had been said to Moses. God's word and his works, if viewed together, will be found mutually to set each other forth. If we make conscience of our duty, we need not question the performance of the promise. But the believer must never put off his armour, or expect lasting peace, till he closes his eyes in death; nay, as his strength and usefulness increase, he may expect more heavy trials; yet the Lord will not permit any enemies to assault the believer till he has prepared him for the battle. Christ Jesus ever lives to plead for his people, and their faith shall not fail, however Satan may be permitted to assault them. And however tedious, sharp, and difficult the believer's warfare, his patience in tribulation may be encouraged by the joyfulness of hope; for he will, ere long, rest from sin and from sorrow in the Canaan above.