Joab returned - The great strength of Rabbah made it hopeless to take it by assault, and the Syrians were not sufficiently broken 2 Samuel 10:15 to make it safe to undertake a regular siege.
Related Commentaries of 2 Samuel 10:14
2 Samuel 10:14
14 And when the children of Ammon saw that the Syrians were fled, then fled they also before Abishai, and entered into the city. So Joab returned from the children of Ammon, and came to Jerusalem.