John Gill Commentary Numbers 21:2

"And Israel vowed a vow unto Jehovah, and said, If thou wilt indeed deliver this people into my hand, then I will utterly destroy their cities." — Numbers 21:2 (ASV)

And Israel vowed a vow to 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 you will 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 6:21Joshua 6:24) and so it is a vow, as Abendana observes, of what they would do when they came to the land of Canaan.

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FOOTNOTES:

• F15: (ytmrxhw) "et anathematisabo", Montanus; "devovebo", Tigurine version.