This is either, first, A command; and then the last clause is an exception from that law, Deuteronomy 20:19, which being delivered by a prophet, might be obeyed. And if this command seem severe, it must be considered that the Moabites were a very wicked people, perfidious, cruel, implacable enemies to God's people upon all occasions, and now in a state of rebellion. Or rather, secondly, A prediction of their success, that they should have so full and complete a victory, that they should be able to do all which is here expressed.
Related Commentaries of 2 Kings 3:19
2 Kings 3:19
19 And ye shall smite every fenced city, and every choice city, and shall fell every good tree, and stop all wells of water, and marb every good piece of land with stones.