John Gill Commentary Numbers 31

And the Lord spoke unto Moses
After the plague upon Israel for their fornication and idolatry, into which they were drawn by the daughters of Moab and Midian, and after the sum of the people was taken in the plains of Moab, and various laws given or repeated, and a little before the death of Moses:

saying ;
as follows.

Avenge the children of Israel of the Midianites
For the injury they had done them, by sending their daughters among them, who enticed them to commit uncleanness with them, and then drew them into the worship of their idols, which brought the wrath of God upon them, and for which 24,000 persons were slain.

Now, though the Moabites had a concern in this affair as well as the Midianites, yet they were spared; which some think was for the sake of Lot, from whom they descended; but why not the Midianites for the sake of Abraham, whose offspring they were by Keturah? Jarchi says, they were spared because of Ruth, who was to spring from them; and so she might, and yet vengeance be taken on great numbers of them: but the truer reason seems to be, either because the sin of the Moabites was not yet full, and they were reserved for a later punishment; or rather because they were not the principal actors in the above affair; but the Midianites, who seem to have advised Balak at first to send for Balaam to curse Israel, and who harboured that soothsayer after he had been dismissed by Balak, and to whom he gave his wicked counsel, and which they readily followed, and industriously pursued:

afterward shalt thou be gathered unto thy people ;
or die, see (Numbers 27:13) , it being some satisfaction to him to see the good land, as he did from Abarim, and the Israelites avenged on their enemies before his death.

And Moses spoke to the people, saying
In obedience to the divine command; this must be supposed to be spoken to the heads or princes of the tribes:

arm some of yourselves to the war :
not the whole body of the militia, 600,000 men and upwards, only some of them, and these choice and select men; and, according to the Jewish writers, good men, who, detesting the sins of lewdness and idolatry, would more strictly and severely avenge themselves on the Midianites for drawing their brethren into those sins, by which they fell; and so Jarchi calls them righteous men:

[and] let them go against the Midianites, and avenge the Lord of
Midian :
what the Lord calls the vengeance of the Israelites, Moses calls the vengeance of the Lord, because they were the Lord's people, and his cause and theirs the same: and because the sins they were drawn into by the Midianites were not only against themselves, and to their prejudice, but against the Lord and to the dishonour of his name.

Of every tribe a thousand So that the whole number of those that were armed were 12,000 as after given:

throughout all the tribes of Israel ; this is observed, as Jarchi thinks, to comprehend the tribe of Levi, which in some cases was left out of the account:

shall you send to the war ; to fight with Midian.

So they were delivered out of the thousands of Israel
] Several thousands very probably offered themselves voluntarily to go to the war, or however were summoned together on this account, and out of them the following number was delivered of picked and chosen men:

a thousand [of every] tribe, twelve thousand armed for war ;
which was but a small number to go against so considerable a nation and people; but this was done to show that the war was of the Lord, he would fight the battle, and give them victory, it being the same to him to do it with few as with many; and to encourage the people of Israel to believe the Lord would give them success against their many and powerful enemies in the land of Canaan, on the borders of which they now were.