Charles Ellicott Commentary 2 Samuel 10:19

"And when all the kings that were servants to Hadarezer saw that they were put to the worse before Israel, they made peace with Israel, and served them. So the Syrians feared to help the children of Ammon any more." — 2 Samuel 10:19 (ASV)

Servants to Hadarezer. —The vassal kings, who had been subject to Hadarezer, now transferred their allegiance to David; but it is not said that Hadarezer himself became subject, though it is clear from 2 Samuel 8:3–7 that he was greatly weakened and suffered the loss of great plunder. From 1 Kings 11:23–24, it is clear that an escaped dependent of Hadarezer maintained himself in the territory of Damascus as an enemy of Israel; it is also stated in 1 Kings 4:21 that Solomon reigned over all kingdoms from the Euphrates to the border of Egypt. It is therefore probable that Hadarezer also acknowledged the suzerainty of David and Solomon.