Matthew Henry Commentary 2 Samuel 8:1-8

"And after this it came to pass, that David smote the Philistines, and subdued them: and David took the bridle of the mother city out of the hand of the Philistines. 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. David smote also Hadadezer the son of Rehob, king of Zobah, as he went to recover his dominion at the River. And David took from him a thousand and seven hundred horsemen, and twenty thousand footmen: and David hocked all the chariot horses, but reserved of them for a hundred chariots. And when the Syrians of Damascus came to succor Hadadezer king of Zobah, David smote of the Syrians two and twenty thousand men. Then David put garrisons in Syria of Damascus; and the Syrians became servants to David, and brought tribute. And Jehovah gave victory to David whithersoever he went. And David took the shields of gold that were on the servants of Hadadezer, and brought them to Jerusalem. And from Betah and from Berothai, cities of Hadadezer, king David took exceeding much brass." — 2 Samuel 8:1-8 (ASV)

David subdued the Philistines. They had long been troublesome to Israel. And after the long and frequent struggles the saints have with the powers of darkness, like Israel with the Philistines, the Son of David shall tread them all under foot and make the saints more than conquerors.

He smote the Moabites and made them tributaries to Israel. Two parts he destroyed, the third part he spared. The line that was to keep alive, even though it was only one, is commanded to be a full line. Let the line of mercy be stretched to the utmost.

He also smote the Syrians. In all these wars David was protected, and for this, in his psalms, he often gives glory to God.