Numbers 21:2

And Israel vowed a vow unto the Lord
The Israelites made supplication to the Lord for help against their enemies, and that he would give them victory over them, and made promises to him:

and said, if thou wilt indeed deliver this people into my hand;
certainly and entirely deliver them, so as that a complete victory shall be obtained over them:

then will I utterly destroy their cities;
or "anathematize", or devote them to utter destruction ^F15; slay man and beast, burn their houses and take their goods, not for a spoil, for their own private use, but reserve them for the service of God; all which is implied in the vow made, as was done to Jericho, ( Joshua 6:21 Joshua 6:24 ) and so it is a vow, as Abendana observes, of what they would do when they came to the land of Canaan.