The war between the houses of David and Saul went on for a long time, but David gradually grew stronger while that of Saul grew weaker. Perhaps we can draw an analogy here between our war against the flesh. When we are born again we do not suddenly become perfect, we still have battles to fight, but over time we become more Christlike, and the flesh becomes weaker.
David had six sons. He also had several wives. Now while in OT times it was common to have more than one wife, it tended to cause problems and is not approved of by the Bible. It was more common on those days because men tended to die in battle and women needed a husband for security. In Genesis the original patterns was one man one woman, a pattern affirmed by Jesus and throughout the New Testament.
These sons did not bring unalloyed joy to David's life. Ammon raped his sister and was killed by Absalom. Absalom rebelled against David. Adonijah tried to take the throne before Solomon.
Abner, leader of Saul's army was falsely accused and changed sides. David accepted this, but also demanded that Saul bring Michal who had been promised to him as a wife by Saul. This may have been simply getting his own back, or maybe he wanted another wife, or it could have been a political ploy to help bring unity (ie the house of Saul and David united).
Abner made preparations for David eventually becoming king.