Now there was a long war between the house of Saul and the house of David; and David grew steadily stronger, but the house of Saul grew weaker continually. [2 Samuel 3:1]
David was anointed king of Israel by God, but at that time only the tribe of Judah acknowledged him as king. The remaining tribes supported the house of Saul as their king. Thus there was a long war between the 2 armies. Eventually the house of Saul was defeated and David became king of a united Israel.
This situation is not unlike the heart of a Christian. A Christian proclaims Jesus as his king, but there might be some aspects of his life that have not surrendered to the King. As such there is a battle between grace and sin, the spirit against the flesh. As the sanctification continues, the flesh (like the house of Saul) becomes weaker and the spirit (like David) becomes stronger. Eventually the believer becomes a perfect man.