Deuteronomy 20:16-20 - Laws Concerning War With Canaanite Nations
16 But of the cities of these people, which Jehovah your God gives you for an inheritance, you shall save alive nothing that breathes. 17 But you shall utterly destroy them: the Hittites, and the Amorites, the Canaanites, and the Perizzites, the Hivites and the Jebusites; as Jehovah your God has commanded you, 18 so that they may not teach you to do according to all their abominations which they have done to their gods. So you would sin against Jehovah your God.
19 When you shall besiege a city a long time in making war against it to take it, you shall not destroy its trees by forcing an axe against them. For you may eat of them, and you shall not cut them down. For is the tree of the field a man that it should go before you to lay siege? 20 Only the trees which you know that they are not trees for food, you shall destroy and cut them down. And you shall build bulwarks against the city that makes war with you, until it is subdued.