https://www.btstories.com/ammonites/

In all of David’s conquest, he didn’t attack the Ammonites because the king had been kind to him when he was running from Saul. One day, the king of the Ammonites died and his son ruled in his place.

David sent a group of men with a message of comfort to the new king. When the men got there, the Ammonite leaders said to their king, “These men aren’t here to show David’s concern for you. They aren’t here to show respect for your father. They’re spying on us. They’re scouting out our city to see where we’re weak.”