Deuteronomy 20:16-20 -

This was for cities at a distance; it was to be otherwise with the cities of the Canaanites. To them no offer of peaceful submission was to be made, and when the city was taken, all the inhabitants without reserve were to be destroyed. This was in accordance with God's command to Israel ( Exodus 23:31-33 ; Exodus 34:11-16 ; Deuteronomy 7:1-3 ), and as a precaution against the risk of the people being seduced into idolatry by the heathen should they be allowed to remain in the land. But whilst engaged in besieging a town, they were not to destroy the fruit trees that were outside the walls; but trees that were not for food they might cut down and use in their operations against the city.