Judges 20:21 - Joseph Benson’s Commentary on the Old and New Testaments

The children of Benjamin came forth out of Gibeah Those forces which were left in Gibeah for its defence issued out upon the Israelites in their rear, while the other Benjamites fought against them in front. And destroyed that day twenty and two thousand Since they were engaged in so good a cause, and God himself bid them go up, it may seem strange that they should receive such a defeat. But it is to be observed, he only bid them go, but did not promise them success. And undoubtedly they were highly blameable that they did not ask counsel of him in such an important matter as going to war with their brethren. For we find they absolutely determined upon it without doing so, and only inquired who should be in the van of their army. The Benjamites certainly deserved punishment. But to engage with them in a civil war was certainly what they ought not to have done without consulting God. It may be, if they had done so, God would have directed them to have sent another message, and that in HIS name, to the Benjamites, which might have had the desired effect, without proceeding to shed the blood of brethren, and exposing their own to be shed by brethren in such an awful manner. Add to this, that these tribes had many and great sins reigning among themselves, and they should not have proceeded to so great a work with polluted hands; but should have pulled the beam out of their own eye, before they attempted to take that out of their brother Benjamin's eye: which, because they did not, God doth it for them, bringing them through the fire, that they might be purged from their dross. And God would hereby show, that the race is not to the swift, nor the battle to the strong. We must never lay that weight on an arm of flesh which only the Rock of ages will bear.