Deuteronomy 20:18-19

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.

19Perhaps you are preparing to attack a city. Your soldiers may have their camp around the city for a long time. If so, you must not cut down the fruit trees. Then you can eat the fruit from them. These trees are not people, like your enemies! So do not attack them!