David’s victory over the Moabites is a fulfillment of the promise and prophecy made in Numbers 24:17–19,1 and it serves to highlight the faithfulness of the Lord.2 When he puts two-thirds of their men to death, he does so to ensure that they will not be a threat to Israel going forward and that they will still be able to provide some kind of tribute.3 The Moabites will remain subject to Israel until the death of Ahab (2 Kings 3:4–5).4
2 And he defeated Moab and he measured them with a line, making them lie down on the ground. Two lines he measured to be put to death, and one full line to be spared. And the Moabites became servants to David and brought tribute.