כי תצא למלחמה, outside the boundaries of the land of Israel. When a war is fought inside the boundaries of the Land of Israel, i.e. a defensive war, no one is allowed to be exempt from army service; even a bridegroom fresh from the wedding canopy is called up. (Sotah 44)

פן ימות במלחמה ואיש אחר יחנכנו, perhaps the man in question had been guilty of a sin on account of which he would be punished by death or one of the punishments mentioned in the Tochahach, the lengthy warnings of Moses in chapter 28. His death would undermine the morale of his brothers in arms.

ופקדו שרי צבאות, after all those who were sent home had left. If even one of the people that should have been sent home were included in the roll call by the commanders this might eventually prove demoralising if the party in question became a casualty of war. Compare Amos 9,1 הך הכפתור וירעשו הספים, “strike a knob and the very foundations will quake,” [insignificant appearing actions have far-reaching consequences. Ed.]

הרחוקות ממך מאד, distant from the present location of the Israelites in their camps. At that time the boundaries of the Land of Israel will be far from the center in every direction.

לא 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.

כי 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.

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

כי 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.

לבא 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.]

רק עץ אשר תדע כי לא עץ מאכל הוא, even if it belongs to a category of trees which normally bear fruit; if you know that it is defective or too old to still bear fruit, so that no sane human being would waste his time on tending it, pruning it, etc. אותו תשחית, such a tree you may destroy for your own purposes.