Matthew Henry Commentary Numbers 31:1-6

"And Jehovah spake unto Moses, saying, Avenge the children of Israel of the Midianites: afterward shalt thou be gathered unto thy people. And Moses spake unto the people, saying, Arm ye men from among you for the war, that they may go against Midian, to execute Jehovah`s vengeance on Midian. Of every tribe a thousand, throughout all the tribes of Israel, shall ye send to the war. So there were delivered, out of the thousands of Israel, a thousand of every tribe, twelve thousand armed for war. And Moses sent them, a thousand of every tribe, to the war, them and Phinehas the son of Eleazar the priest, to the war, with the vessels of the sanctuary and the trumpets for the alarm in his hand." — Numbers 31:1-6 (ASV)

All who, without commission from God, dare to execute private revenge, and who, from ambition, covetousness, or resentment, wage war and desolate kingdoms, must one day answer for it. But if God, instead of sending an earthquake, a pestilence, or a famine, is pleased to authorize and command any people to avenge his cause, such a commission surely is just and right.

The Israelites could show such a commission, though no persons now can do so. Their wars were begun and carried on expressly by Divine direction, and they were enabled to conquer by miracles.

Unless it can be proved that the wicked Canaanites did not deserve their doom, objectors only prove their dislike of God and their love for his enemies. Humankind makes light of the evil of sin, but God abhors it. This explains the terrible executions of the nations that had filled the measure of their sins.