Charles Ellicott 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)

He smote Moab. —David’s former friendly relations with Moab (probably connected with his own descent from Ruth) are mentioned in 1 Samuel 22:3–4. The cause of his entire change of attitude towards them is not certainly known, but according to Jewish tradition, the Moabites had betrayed their trust and had put to death David’s father and mother. This is not unlikely, as his parents are never mentioned again after they were left in Moab.

Others think that the Moabites had been guilty of some treachery towards David in his war with the Syrians and Ammonites. The two suppositions are quite consistent, and both may have been true. Many writers see in this conquest at least a partial fulfillment of the prophecy in Numbers 24:17.

With two lines. —This expression with the “one full line” of the next clause is equivalent to saying that David measured off the bodies of his prostrate enemies with a line divided into three equal parts. When they had been made to lie down on the ground, side by side, the line was stretched over them.

Those found under the first two parts of it were put to death, while those under the third part were spared; thus, two-thirds of all the Moabite men perished. There is no mention of this in 1 Chronicles 18:2.

Brought gifts. —A frequent euphemism for paid tribute. (Compare 2 Samuel 8:6).