Albert Barnes 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 - This gives us an idea of the great power of Hadarezer, and consequently of the strength of Israel during David’s victorious reign.