John Gill Commentary Numbers 31:3

"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." — Numbers 31:3 (ASV)

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.