Charles Ellicott 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)

When Joab saw. —The keen eye of this experienced general immediately perceived both the advantages and the danger of the enemy's formation. He threw his whole force between their two divisions, organizing his own army into two parts, one facing the Ammonites and the other the Syrians, but each part was capable of supporting the other if needed. The enemy was thus cut in two, while the Israelites formed one compact body. He himself took command of the wing facing the Syrians with the choice troops of Israel, since he had the stronger enemy to face, while he placed the rest of the forces opposing the Ammonites under the command of his brother Abishai.