Numbers 21:2

2.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 to encourage themselves by entreating His aid. To this slate of submissiveness they had been subdued by the chastisements of God, although the continuance of their obedience, as we shall presently see, was not of long duration. Any one at first sight would say that there was something absurd in this vow; but we gather from the result, that it was lawful and approved by God; for the sign of His approbation was that tie hearkened to the vows and cry of the people. I admit, indeed, that God sometimes answers defective prayers, but there is no doubt whatever but 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 sweet savor; and the reply to this question will be easy, if we bear in mind that the vow did not originate in inconsiderate 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 appointed the Israelites to execute His sentence. Hence the vow, of which we are now treating, was not idly spoken, being founded on God’s word, which is the first rule for vowing rightly. It was, indeed, allowable for them to spare the cities, in order to possess them themselves; but 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. This at any rate we must conclude, that although God had not openly and expressly commanded the cities to be utterly destroyed, still this vow was dictated by the Holy Spirit, lest the people should yield to sloth, and set themselves down in a single corner, but that, having desolated and wasted this region, they might encourage themselves the more to further progress. The vero חרם charam, which Moses employs, signifies, indeed, to destroy, and from it is derived the word, חרמה chormah, or Hormah, which implies a species of anathema, as if they devoted the land to the curse of God. Moses, however, adds, that the people performed the vow, under the obligation of which they had laid themselves; and praiseworthy indeed was their magnanimity, in refusing to avail themselves of a comfortable home by destroying the cities, which they had acquired by the right of war.

We know not whether the cities were destroyed immediately after the victory over their enemies; indeed, I rather conjecture that there was some interval of time, because the people did not straightway enter the boundaries of the promised land. And this more clearly appears from chapter 33, where, after this battle was fought, certain stations are enumerated, which are in another direction. It is probable, therefore, that they fought outside the boundaries of the Canaanites, and that, when the people came here soon afterwards, the land was finally put to the sword.