Vex the Midianites, and smite them. Inasnmch as God constantly forbids His people to take vengeance, it is surprising that the people of Israel should now be instigated to do so; as if they were not already more than enough disposed to it. We must bear in mind, however, that since God, who is the just avenger of all wickedness, often makes use of men's instrumentality, and constitutes them the lawful ministers for the exercise of his vengeance, it must not be altogether condemned without exception, but only such vengeance as men themselves are impelled to by carnal passions. If any one is injured, straightway he is carried away to the desire of vengeance by the stimulus of his own private injury; and this is manifestly wrong: but if a person is led to inflict punishment by a just and well-regulated zeal towards God, it is not his own cause, but that of God which he undertakes. God did not, therefore, desire to give reins to His people's anger, so as to repay the Midianites as they had deserved in the violence of its impulse; but He armed them with His own sword for their punishment; as if He had declared that there was a just cause for their war, and that they need not fear the charge of cruelty, if they exterminated such obnoxious enemies. For, although Balaam alone had imagined this snare, still the guilt is laid upon the whole people. In the meantime, the punishment of the Moabites is delayed, although they had apparently inflicted the grosser injury. Because no good reason here appears why God should mercifully bear with the one nation, whilst He hastens speedily to the punishment of the others, let us learn to regard His judgments with reverence, and not to presume to discuss them further than is lawful. Let it be sufficient for us to know that war was justly declared against the Midianites, because it was not their fault that Israel was not ruined by their iniquitous impiety.

Vex the Midianites, etc. - See this order fulfilled, Numbers 31:1-20. Twelve thousand Israelites attacked the Midianites, destroyed all their cities, slew their five kings, every male, and every grown up woman, and took all their spoils.

Vex the Midianites, and smite them. Go to war with them, and smite them with the sword; not the Moabites, but the Midianites, though they were both confederates against Israel; but God had given a charge not to contend in battle with Moab, Deuteronomy 2:9, they were spared for the sake of Lot, from whom they sprang; and, as Jarchi says, for the sake of Ruth, who was to come from them; and so in the Talmud (l); though they did not entirely escape the divine resentment, as appears from Deuteronomy 23:3 but the Midianites were the first that advised to send for Balaam, and with them he stayed and was entertained, after he had been dismissed by Balak; and it seems as if it was to them he gave the wicked counsel, to draw Israel into fornication, and so into idolatry, and thereby bring the curse of God upon them; which advice they communicated to the Moabites, and both were concerned in putting it into execution; see Numbers 22:4.
(l) T. Bab. Bava Kama, fol. 38. 2.

Vex the Midianites, and smite them--They seem to have been the most guilty parties. (Compare Numbers 22:4; Numbers 31:8).

The Midianites - And why not the Moabites. It is probable the Midianites were most guilty, as in persuading Balak to send for Balaam, Numbers 22:4, Numbers 22:7. So in the reception of Balaam after Balak had dismissed him, Numbers 31:8, and in farther consultation with him, and in contriving the means for the executing of this wicked plot.