Nahash king of the Ammonites died, and his son succeeded him as king. David thought, "I will show kindness to Hanun son of Nahash, because his father showed kindness to me." So David sent a delegation to express his sympathy to Hanun concerning his father. When David's men came to Hanun in the land of the Ammonites to express sympathy to him, the Ammonite nobles said to Hanun, "Do you think David is honoring your father by sending men to you to express sympathy? Haven't his men come to you to explore and spy out the country and overthrow it?" So Hanun seized David's men, shaved them, cut off their garments in the middle at the buttocks, and sent them away. Well, It's good to be cautious, but I think they went a little too far.
When someone came and told David about the men, he sent messengers to meet them, for they were greatly humiliated. The king said, "Stay at Jericho till your beards have grown, and then come back." When the Ammonites realized that they had become a stench in David's nostrils, Hanun and the Ammonites sent a thousand talents [a] of silver to hire chariots and charioteers from Aram Naharaim, [b] Aram Maacah and Zobah. They hired thirty-two thousand chariots and charioteers, as well as the king of Maacah with his troops, who came and camped near Medeba, while the Ammonites were mustered from their towns and moved out for battle. On hearing this, David sent Joab out with the entire army of fighting men. The Ammonites came out and drew up in battle formation at the entrance to their city, while the kings who had come were by themselves in the open country. 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. 47000 people! he must of been super blessed by God.
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. All because of that one thing they did.