The battle was set against him before and behind. — Literally, the front of the battle had become towards him, front and rear. The order of words is different in Samuel, and a preposition added (“on front and on rear”). The Ammonites lay in front of the city, their Aramæan allies at some distance away, in the plain. For Joab to attack either with his entire army would have been to expose his rear to the assault of the other. He therefore divided his forces.
The choice of Israel. — Literally, the chosen or young warriors (singular collective) in Israel (i.e., in the Israelitish army). These Joab himself led against the Aramæans, as the most dangerous enemy, while he sent a detachment, under his brother Abishai, to cope with the Ammonites.
Put them in array. — Rather, set the battle in array, or drew up against... (1 Chronicles 19:17; 1 Chronicles 12:33). The same Hebrew phrase recurs in 1 Chronicles 19:11.