When the Ammonites saw that the Arameans had fled, they too fled before Abishai, and they entered the city. So Joab returned from fighting against the Ammonites and came to Jerusalem.
Here we have, I. The preparation which the Ammonites made for war, Sa2 10:6. They saw they had made themselves very odious to David and obnoxious to his just displeasure. This they might easily have foreseen when they abused his ambassadors, which was no other than a challenge to war, and a bold defiance of him.
Commenting on 2 Samuel 10:6-14
And when the children of Ammon saw that the Syrians were fled,.... In whom they put great confidence: then they fled also before Abishai; without engaging at all with him, as it seems: and entered into the city; out of which they came, either Rabbah or Medeba, for their safety: so Joab returned from the children of Ammon; did not stay to lay siege to...
So Joab returned and came to Jerusalem--Probably the season was too far advanced for entering on a siege.