Only the trees which thou knowest that they be not trees for meat,.... Which might be known not only by their not having fruit upon them, but by other tokens, and even at a time of year when there was no fruit on any, which might be sometimes the season of a siege:

thou shalt destroy and cut them down; if so to do was of any disservice to the enemy, or of any service to them, as follows; they had a liberty to destroy them if they would:

and thou shall build bulwarks against the city that maketh war, until it be subdued; build bulwarks of the trees cut down, and raise batteries with them, or make machines and engines of the wood of them, to cast stones into the city to annoy the inhabitants of it, in order to make them surrender, and until they do it. All this may be an emblem of the axe being to be laid to fruitless trees in a moral and spiritual sense; and of trees of righteousness, laden with the fruits of righteousness, the planting of the Lord, being preserved and never to be cut down or rooted up; see Matthew 3:10.

"Only the trees which thou knowest that they are not trees of eating (i.e., do not bear edible fruits), mayest thou hew down, and build a rampart against the town till it come down," i.e., fall down from its eminence. For ירד as applied to the falling or sinking of lofty fortifications, see Deuteronomy 28:52; Isaiah 32:19. מצור, compressing or forcing down; hence, as applied to towns, במּצור בּוא, to come into siege, i.e., to be besieged (Deuteronomy 20:19; 2 Kings 24:10; 2 Kings 25:2). In Deuteronomy 20:20 it is used to denote the object, viz., the means of hemming in a town, i.e., the besieging rampart (cf. Ezekiel 4:2).

Only the trees which thou knowest that they be not trees for meat, thou shalt destroy and cut them down; and thou shalt build bulwarks against the city that maketh war with thee, until it be subdued.

it...: Heb. it come down

20. thou shalt build bulwarks against the city that maketh war with thee-It is evident that some sort of military engines were intended; and accordingly we know, that in Egypt, where the Israelites learned their military tactics, the method of conducting a siege was by throwing up banks, and making advances with movable towers, or with the testudo [Wilkinson].

20:10-12 The Israelites are here directed about the nations on whom they made war. Let this show God's grace in dealing with sinners. He proclaims peace, and beseeches them to be reconciled. Let it also show us our duty in dealing with our brethren. Whoever are for war, we must be for peace. Of the cities given to Israel, none of their inhabitants must be left. Since it could not be expected that they should be cured of their idolatry, they would hurt Israel. These regulations are not the rules of our conduct, but Christ's law of love. The horrors of war must fill the feeling heart with anguish upon every recollection; and are proofs of the wickedness of man, the power of Satan, and the just vengeance of God, who thus scourges a guilty world. But how dreadful their case who are engaged in unequal conflict with their Maker, who will not submit to render him the easy tribute of worship and praise! Certain ruin awaits them. Let neither the number nor the power of the enemies of our souls dismay us; nor let even our own weakness cause us to tremble or to faint. The Lord will save us; but in this war let none engage whose hearts are fond of the world, or afraid of the cross and the conflict. Care is here taken that in besieging cities the fruit-trees should not be destroyed. God is a better friend to man than he is to himself; and God's law consults our interests and comforts; while our own appetites and passions, which we indulge, are enemies to our welfare. Many of the Divine precepts restrain us from destroying that which is for our life and food. The Jews understand this as forbidding all wilful waste upon any account whatsoever. Every creature of God is good; as nothing is to be refused, so nothing is to be abused. We may live to want what we carelessly waste.