Albert Barnes Commentary 2 Samuel 8:2

"And he smote Moab, and measured them with the line, making them to lie down on the ground; and he measured two lines to put to death, and one full line to keep alive. And the Moabites became servants to David, and brought tribute." — 2 Samuel 8:2 (ASV)

David took great numbers of Moabites as prisoners of war. He made them lie down on the ground, divided them by a measuring line into three parts, put two-thirds to death, and spared one-third.

The cause of this war and of such severe treatment is not known, especially since the Moabites had previously been very friendly with David (1 Samuel 22:3–4).

However, it seems likely from the tone of Psalm 60:1-12 that David had met with a temporary setback in his Syrian wars. The Moabites and Edomites may have treacherously taken advantage of the situation and perhaps tried to cut off his retreat.