Albert Barnes 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)

Avenge the Lord of Midian - The war against the Midianites was no ordinary war. It was indeed less a war than the execution of a divine sentence against a most guilty people.

Doubtless there were many among the Midianites who were personally guiltless as regards Israel. But the rulers deliberately adopted the counsel of Balaam against Israel, and their commands had been all too readily obeyed by their subjects. The sin therefore was national, and the retribution could be no less so.

But the commission of the Israelites in the text must not be understood as a general license to kill. They had no discretion to kill or to spare. They were commanded to exterminate without mercy, and were brought back to their task (Numbers 31:14) when they showed signs of flinching from it.

In this and similar matters, they had no alternative except to fulfill the commands of God. This was a terrible but doubtless salutary manifestation, as was the later slaughter of the Canaanites, of God’s wrath against sin; and a type of the future extermination of sin and sinners from His kingdom.