John Calvin 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 unto the Lord. This was a manifestation of piety: when they had sustained a loss, not to cast away hope, nor to murmur against God; but rather to encourage themselves by entreating His aid. To this state of submissiveness they had been subdued by God's chastisements, although the continuation of their obedience, as we shall soon see, did not last long.

Anyone at first sight would say that there was something absurd in this vow. However, we gather from the result that it was lawful and approved by God, for the sign of His approval was that He listened to the vows and cry of the people. I admit, indeed, that God sometimes answers defective prayers, but there is no doubt whatever that Moses here commends their piety in the vow.

We must consider, then, how it was lawful for them to offer the destruction of cities and the wasting of lands to God as a sacrifice of a sweet savor. The reply to this question will be easy if we bear in mind that the vow did not originate in thoughtless zeal, but rather in the command of God.

It seems cruel to destroy an entire nation, but God had not only decreed its destruction but had also appointed the Israelites to execute His sentence. Therefore, the vow we are now discussing was not idly spoken, as it was founded on God’s word, which is the primary rule for vowing rightly.

It was, indeed, allowable for them to spare the cities to possess them themselves. However, it was also allowable to devote them as an offering (in anathema) of first-fruits to God, as we are elsewhere told of the city of Jericho.

At any rate, we must conclude that even though God had not openly and expressly commanded the cities to be utterly destroyed, this vow was still dictated by the Holy Spirit. This was so that the people would not yield to laziness and settle down in a single corner; but rather, that after desolating and wasting this region, they might be more encouraged to make further progress.

The Hebrew term חרם charam, which Moses employs, indeed signifies to destroy, and from it is derived the word חרמה chormah, or Hormah, which implies a type of anathema, as if they devoted the land to God’s curse.

Moses, however, adds that the people performed the vow they had undertaken. Praiseworthy indeed was their magnanimity in refusing to take advantage of a comfortable home by destroying the cities they had acquired by the right of war.

We do not know whether the cities were destroyed immediately after the victory over their enemies. Indeed, I rather conjecture that there was some interval, because the people did not immediately enter the boundaries of the promised land. This appears more clearly from Numbers 33, where, after this battle was fought, certain encampments are listed that are in another direction. It is probable, therefore, that they fought outside the Canaanite boundaries and that, when the people came to this area soon afterwards, the land was finally put to the sword.