That they teach you not to do after all their abominations, which they have done unto their gods; so should ye sin against the LORD your God.

Matthew Henry's Concise Commentary

20:10-12 The Israelites are here directed about the nations on whom they made war. Let this show God's grace in dealing with sinners. He proclaims peace, and beseeches them to be reconciled. Let it also show us our duty in dealing with our brethren. Whoever are for war, we must be for peace. Of the cities given to Israel, none of their inhabitants must be left. Since it could not be expected that they should be cured of their idolatry, they would hurt Israel. These regulations are not the rules of our conduct, but Christ's law of love. The horrors of war must fill the feeling heart with anguish upon every recollection; and are proofs of the wickedness of man, the power of Satan, and the just vengeance of God, who thus scourges a guilty world. But how dreadful their case who are engaged in unequal conflict with their Maker, who will not submit to render him the easy tribute of worship and praise! Certain ruin awaits them. Let neither the number nor the power of the enemies of our souls dismay us; nor let even our own weakness cause us to tremble or to faint. The Lord will save us; but in this war let none engage whose hearts are fond of the world, or afraid of the cross and the conflict. Care is here taken that in besieging cities the fruit-trees should not be destroyed. God is a better friend to man than he is to himself; and God's law consults our interests and comforts; while our own appetites and passions, which we indulge, are enemies to our welfare. Many of the Divine precepts restrain us from destroying that which is for our life and food. The Jews understand this as forbidding all wilful waste upon any account whatsoever. Every creature of God is good; as nothing is to be refused, so nothing is to be abused. We may live to want what we carelessly waste.