Albert Barnes Commentary 2 Samuel 10:14

"And when the children of Ammon saw that the Syrians were fled, they likewise fled before Abishai, and entered into the city. Then Joab returned from the children of Ammon, and came to Jerusalem." — 2 Samuel 10:14 (ASV)

Joab returned. An assault on Rabbah was hopeless due to its great strength, and a regular siege was unsafe because the Syrians were not yet sufficiently broken (2 Samuel 10:15).