When Hadadezer’s subjects saw that they had been defeated by Israel, they made peace with David and became subject to him. So the Arameans were unwilling to help the Ammonites anymore.
KJV
And when the servants of Hadarezer saw that they were put to the worse before Israel, they made peace with David, and became his servants: neither would the Syrians help the children of Ammon any more.
ASV
And when the servants of Hadarezer saw that they were put to the worse before Israel, they made peace with David, and served him: neither would the Syrians help the children of Ammon any more.