Absalom had appointed Amasa over the army in place of Joab. Amasa was the son of a man named Ithra, the Ishmaelite who had married Abigail, the daughter of Nahash and sister of Zeruiah the mother of Joab.
Jewish Publication Society of America Version (JPS1917) - 2 Samuel 17:25 >And Absalom had set Amasa over the host instead of Joab. Now Amasa was the son of a man, whose name was Ithra the Jesraelite, that went in to Abigal the daughter of Nahash, sister to Zeruiah Joab's mother.