Deuteronomy 20:15-18

Do this to all the cities that are far away, that do not belong to the nations nearby. But leave nothing alive in the cities of the land the LORD your God is giving you. Completely destroy these people: the Hittites, Amorites, Canaanites, Perizzites, Hivites, and Jebusites, as the LORD your God has commanded. Otherwise, they will teach you what they do for their gods, and if you do these hateful things, you will sin against the LORD your God.