As the war between Israel's 2 kings waged on, David's side became increasingly stronger. In fact, David's strength increased to the point that the opposing general, Abner, defected to David's side. It is true that he had an excuse to switch sides. lshbosheth rebuked Abner for taking one of Saul's former mistresses for himself. That was enough of a pretext for Abner to contact David with an offer of peace.
Now, David had some simple terms for the ensuing peace. He wanted his former wife Michal back (remember, she was a princess). It seems likely that David wanted her back more for political reasons than for romantic ones. After all, at this point he had 6 other wives, I'm not sure a seventh one was necessary for David's heart or body to be satisfied. Whatever his reasons, lshbosheth and Abner did force Michal's second husband to surrender her back to David (Saul had wrongly given her to Phaltiel while David was away - I Samuel 25:44).
While Abner was going about to bring the whole nation under David's control, David's nephew Joab was going about to avenge the death of his brother Asahel. Since Abner had killed Asahel away from a battlefield and while he was unarmed, Joab considered it to be murder and felt justified in killing Abner. Incidentally, the later part of this chapter indicates that David did not concur with Joab's judgment. Anyway, Joab tricked Abner into stepping outside of the gate and away from the safety of the city of refuge, and there, Joab killed him.