Being sensible of their own weakness, they endeavour to engage God to help them in the war, which they intended to renew. I will utterly destroy their cities; I will reserve no person nor thing for my own use, but devote them all to total destruction, which was the consequent of such vows. See Leviticus 27:29 Deuteronomy 13:15
Related Commentaries of Numbers 21:2
Numbers 21:2
2 And Israel vowed a vow unto the LORD, and said, If thou wilt indeed deliver this people into my hand, then I will utterly destroy their cities.