1 Chronicles 19:17-19

New International Version

¹⁷ When David was told of this, he gathered all Israel^(A) 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. ¹⁸ But they fled before Israel, and David killed seven thousand of their charioteers and forty thousand of their foot soldiers. He also killed Shophak the commander of their army.

¹⁹ When the vassals of Hadadezer saw that they had been routed by Israel, they made peace with David and became subject to him.

So the Arameans were not willing to help the Ammonites anymore.