Avenge the LORD of Midian— What is called avenging the children of Israel, in Numbers 31:2 is here called avenging the LORD; because the war was with idolaters, the enemies of the true religion, and for the sake of God's peculiar people. Besides, the Midianites were particularly displeasing to God for having seduced the Israelites to the worship of idols; so that to avenge the one was to avenge the other. Onkelos renders this, to avenge the people of the Lord of Midian. See Ainsworth and Poole.
Related Commentaries of Numbers 31:3
Numbers 31:3
3 And Moses spake unto the people, saying, Arm some of yourselves unto the war, and let them go against the Midianites, and avenge the LORD of Midian.