Deuteronomy 20:16-20 - Laws Concerning War With Canaanite Nations
16 Only of the cities of these peoples which Jehovah thy God gave to thee an inheritance, thou shalt not preserve alive and breathing: 17 For destroying, thou shalt destroy them, the Hittite, and the Amorite, the Canaanite, and the Perizzite, the Hivite, and the Jebusite, as Jehovah thy God commanded thee: 18 So that they shall not teach you to do according to all their abominations to their gods; and ye sinned to Jehovah your God.
19 When thou. shalt besiege against a city many days to war against it, to capture it, thou shalt not destroy its wood to thrust an axe upon it; for from it thou shalt eat: and thou shalt not cut it down; for the man the tree of the field, to go from before thee into the fortress. 20 Only the tree which thou shalt know that it is not a tree of food, thou shalt destroy it, and cut down, and build a fortress against the city which made way with thee, till it came down.