He prudently falls upon them first, because they were but mercenaries, and not concerned so much in the success as the Ammonites were, all whose interest lay at stake; and therefore not likely to venture too far in their defence.
Related Commentaries of 2 Samuel 10:13
2 Samuel 10:13
13 And Joab drew nigh, and the people that were with him, unto the battle against the Syrians: and they fled before him.