2 Samuel 10:19

And when all the kings [that were] servants to Hadarezer
That were subject to him, and at his beck and command, at least were hired by him into his service:

saw that they were smitten before Israel;
could not stand their ground against them, being too powerful for them;

they made peace with Israel, and served them;
became tributaries to them, whereby the promise of the land of Canaan made to Abraham and his seed, as reaching to the river Euphrates, had its accomplishment, ( Genesis 15:18 ) ;

so the Syrians feared to help the children of Ammon any more;
who standing by themselves, they were soon and easily conquered by David's forces, as recorded in the following chapters.