Deuteronomy 20:18

14 But the women, children, livestock, and whatever else is in the city—all its spoil—you may take as plunder, and you shall use the spoil of your enemies that the LORD your God gives you.

15 This is how you are to treat all the cities that are far away from you and do not belong to the nations nearby.

16 However, in the cities of the nations that the LORD your God is giving you as an inheritance, you must not leave alive anything that breathes.

17 For you must devote them to complete destruction—the Hittites, Amorites, Canaanites, Perizzites, Hivites, and Jebusites—as the LORD your God has commanded you,

18 so that they cannot teach you to do all the detestable things they do for their gods, and so cause you to sin against the LORD your God.

19 When you lay siege to a city for an extended time while fighting against it to capture it, you must not destroy its trees by putting an axe to them, because you can eat their fruit. You must not cut them down. Are the trees of the field human, that you should besiege them?

20 But you may destroy the trees that you know do not produce fruit. Use them to build siege works against the city that is waging war against you, until it falls.

Read all of Deuteronomy 20