Why David Didn't Go to War with Ammon

In Sefer Shmuel (The Book of Samuel) 21:17, David is almost killed in battle, so the people ask him to stay behind in Jerusalem next time while they go out and fight, lest “The Light of Israel,” i.e. David, is extinguished.  This is further evidence that the events in chapter 21 occurred at the beginning of David’s reign, as this would explain why, back in chapter 10, David did not go out to war against the Ammonites with the rest of the army.