Deuteronomy 20:15-18

This is how you are to treat all the cities that are at a distance from you and do not belong to the nations nearby. However, in the cities of the nations the LORD your God is giving you as an inheritance, do not leave alive anything that breathes. Completely destroy them—the Hittites, Amorites, Canaanites, Perizzites, Hivites and Jebusites—as the LORD your God has commanded you. Otherwise, they will teach you to follow all the detestable things they do in worshiping their gods, and you will sin against the LORD your God.