Deuteronomy 20:16-20 - Laws Concerning War With Canaanite Nations
16 But of the cities of these peoples which Yahweh thy God is giving unto thee as an inheritance, shalt thou not save alive, any breathing thing. 17 But thou shalt devote them to destruction - the Hittites and the Amorites the Canaanites and the Perizzites, the Hivites, and the Jebusites, - as Yahweh thy God hath commanded thee: 18 lest they teach you to do, according to all their abominations, which they have done unto their gods, - and so ye sin against Yahweh your God.
19 When thou shalt besiege a city for many days to fight against it, to take it, thou shalt not destroy the trees thereof by wielding against them an axe, when of them, thou mightest eat, them, there-fore shalt thou not cut down, - for, Is the tree of the field, a man, that it should enter, because of thee into the siege? 20 Nevertheless, the tree of which thou knowest that it is, not a tree yielding food, the same, mayest thou destroy, and cut down, - and so build up a siege-work against the city which is making war with thee, until thou hast subdued it.