Absalom set Amasa over the army in place of Joab. Now Amasa was the son of a man whose name was Ithra the Israelite, who went in to Abigail the daughter of Nahash, sister of Zeruiah, Joab’s mother. (2 Samuel 17:25 NASB)
Have you ever listened to some guy’s story about somebody he knows, and after getting lost in the details realize he has no idea who this guy is either? The basic issue here is that Abigail’s and Zeruiah’s father wasn’t Nahash but Jesse (or was supposed to be). That made Joab David’s nephew, which may be the only reason David let him live.