When the Ammonites saw that the Arameans were fleeing, they too fled before his brother Abishai and went inside the city. So Joab went back to Jerusalem.
After the Arameans saw that they had been routed by Israel, they sent messengers and had Arameans brought from beyond the River, with Shophach the commander of Hadadezer's army leading them.
References for 1 Chronicles 19:16
When David was told of this, he gathered all Israel1 and crossed the Jordan; he advanced against them and formed his battle lines opposite them. David formed his lines to meet the Arameans in battle, and they fought against him.
References for 1 Chronicles 19:17
But they fled before Israel, and David killed seven thousand of their charioteers and forty thousand of their foot soldiers. He also killed Shophach the commander of their army.
When the vassals of Hadadezer saw that they had been defeated by Israel, they made peace with David and became subject to him. So the Arameans were not willing to help the Ammonites anymore.