David took great numbers of the Moabites prisoners of war, and made them lie down on the ground, and then divided them by a measuring line into three parts, putting two-thirds to death, and saving alive one-third. The cause of the war with the Moabites, who had been very friendly with David 1 Samuel 22:3-4, and of this severe treatment, is not known. But it seems likely, from the tone of Psalms 60:1-12 that David had met with some temporary reverse in his Syrian wars, and that the Moabites and Edomites had treacherously taken advantage of it, and perhaps tried to cut off his retreat.
Related Commentaries of 2 Samuel 8:2
2 Samuel 8:2
2 And he smote Moab, and measured them with a line, casting them down to the ground; even with two lines measured he to put to death, and with one full line to keep alive. And so the Moabites became David's servants, and brought gifts.