Deuteronomy 20:16-18

16You will take for yourselves the cities in the land that the Lord your God is giving to you as your home. In those cities, you must kill everything that is alive, people and animals.

17You must completely destroy all the Hittites, the Amorites, the Canaanites, the Perizzites, the Hivites and the Jebusites. The Lord your God has commanded you to do this to the people of those nations.

18If not, they would teach you all the evil things that they do to worship their gods. They would cause you to do bad things against the Lord your God.