Deuteronomy 20:16-20 - Laws Concerning War With Canaanite Nations
16 But in the cities of these nations which the LORD thy God giveth thee to inherit, thou shalt save alive nothing that breatheth. 17 But shalt destroy them without redemption, both the Hittites, the Amorites, the Canaanites, the Perezites, the Hivites and the Jebusites, as the LORD thy God hath commanded thee, 18 that they teach you not to do after all their abominations which they do unto their gods, and so should sin against the LORD your God.
19 When thou hast besieged a city long time in making war against it to take it, destroy not the trees thereof, that thou wouldest thrust an axe unto them. For thou mayest eat of them, and therefore destroy them not. For the trees of the fields are no men that they might come against thee to besiege thee. 20 Neverthelater those trees which thou knowest that men eat not of them, thou mayest destroy and cut them down and make bulwarks against the city that maketh war with thee, until it be overthrown.