Ibn Ezra on Numbers 31:3

החלצו. חגרו חלוציכם: ARM YE MEN. The meaning of hechaletzu (arm ye men) is, gird your men of war.¹Gird a sword upon your men of war (Krinsky). Or, gird the loins of your men of war. Vat. Ebr. 38 reads gird your loins. See Job 38:3 and Is. 5:27. The word hechaletzu can also be rendered as strip. Hence I.E.’s comment (Meijler).

נקמת ה'. היא נקמת בני ישראל בעבור עבודת כוכבים: TO EXECUTE THE LORD’S VENGEANCE. The reference is to Israel’s vengeance²Verse 2 says, Avenge the children of Israel of the Midianites. However, verse 3 states, to execute the Lord’s vengeance on Midian. Hence I.E.’s comment. because they were enticed³By the Midianites. See Num. 25:6 and Num. 31:15,16. into worshipping idols.