John Calvin Commentary Numbers 31:7

"And they warred against Midian, as Jehovah commanded Moses; and they slew every male." — Numbers 31:7 (ASV)

And they warred against the Midianites. It was a notable example of obedience that 12,000 men did not refuse to engage in a war full of danger. It would have been reasonable for them to object that it was not right for them to be exposed to butchery, so to speak, while the rest of the people sat idly in the camp—people who, by their great numbers and with little trouble, could have routed and overcome the enemy.

It was therefore a significant proof of piety that they obeyed God’s command and sought no pretext to cover their cowardice.

God, too, showed by the outcome that He did not recklessly expose His servants to danger, for it is in His power to rescue those whom He takes under His protection, from a hundred deaths. Hence, we are also taught that there is no surer means of safety than to follow where He leads.

What Moses later adds serves to praise their perseverance. With one exception, they were right in killing all the males, including the kings, whom Moses reports were slain in the general slaughter; they were also particularly right to inflict punishment on Balaam, who by his cunning and snares had endeavored to destroy the people of God.

They were also right in plundering the whole land; nor did they act with less appropriateness and wisdom in razing all the cities and towns, which might have tempted the timid and inactive to settle there. For, as we have seen before, all hindrances were to be removed so that the people might advance freely and without obstruction into the land of Canaan. Otherwise, if an opportunity for rest had presented itself, many would have willingly forgone the promised inheritance.

Therefore, the cities were consumed by fire, so that they would not provide any reason for those who were willing to stay. Up to this point, the selected soldiers faithfully performed their duty. However, in one respect they failed: driven by either greed or lust, they preserved the women alive. We will discuss this point more shortly.