Abner eventually gave up and surrendered to David.  He even offered to help transfer the remaining parts of Israel to David.  However, he was not able to do it because Joab found him and killed him, to avenge the death of his brother Asahel.  Later, David attacked Jerusalem, which was controlled by Jebusites.  David said that the first man who went up to attack the city would be put in charge of his army.  Joab went first, so he became commander.