Sforno on Deuteronomy 20:19/ספורנו על דברים כ׳:י״ט

1לא תשחית את עצה לנדוח עליו גרזן, do not destroy its trees merely in order to practice wielding an axe. Destruction must not be wanton; it is justified only if it serves to harm the enemy residing within the city.

2כי ממנו תאכל, wanton destruction of such trees is justified only when it is not certain that without such action the war will come to a successful conclusion. The wars of conquest of the Land of Israel, however, the success of which has been guaranteed by G’d, does not fall into this category; it will be won without the need to destroy such trees. Destroying fruit bearing trees in the Land of Israel harms the country.

3כי ממנו תאכל, because as soon as you have conquered the land you yourselves will want to eat the fruit of such trees.

4כי האדם עץ השדה, for is a tree of the field equivalent to a human being, capable of defending itself and therefore posing a danger to you? Neither is it able to surrender on account of the siege; seeing that this is so, even though part of its timber could serve as a rampart for helping you to mount an attack against the city itself, since this will not be achieved (directly) by cutting down these trees it is not proper for you to destroy such trees, as opposed to your being permitted to kill human beings in that city opposing you and endangering you.

5לבא מפניך במצור, so that you would besiege the city on their account so they would surrender to you? [the underlying idea, if I understand the author and the text correctly, is that the purpose of laying siege to a city instead of assaulting it and destroying it, is to preserve it intact after its inhabitants have been forced to surrender. Cutting down fruit bearing trees would be the opposite of your objectives in such a war. Ed.]