Albert Barnes Commentary 2 Samuel 10:9

"Now when Joab saw that the battle was set against him before and behind, he chose of all the choice men of Israel, and put them in array against the Syrians:" — 2 Samuel 10:9 (ASV)

The two armies of the Ammonites and the Syrians were drawn up facing one another. The Ammonites were supported by the city of Rabbah behind them, and the Syrians were in great force with numerous chariots able to maneuver in the plain in front of Medeba. If Joab advanced against either army, he would have the other in his rear.