From New World Encyclopedia
Unification Aspects:

Abner, the son of Ner, was one of the two great military leaders in the earliest history of the Israelite monarchy, along with David's commander, Joab. Although his role in Saul's army is not given in much detail, there can be no doubt that Abner was instrumental in Saul's military successes. Abner cannot be blamed for siding with Saul against David, for Saul was the anointed king, and we have no indication that Abner knew that God had chosen David until much later in the story.

Indeed, even after Saul was slain at Gilboa, David must have appeared to Abner as a usurper who had even allied himself with the Philistines. Yet, Abner must have had doubts about his cousin (or nephew) Saul's behavior toward David. The young champion had fought valiantly for Saul's cause and won the king's confidence until an "evil spirit from the Lord" moved Saul to try to kill David.

Abner naturally rallied his forces to Ish-bosheth's cause after Saul's death, uniting the northern tribes, while David reigned over the single tribe of Judah at Hebron. We do not know the truth of the charges that Abner slept with one of Saul' concubines, but it is clear that by this time he had grown disaffected with Ish-bosheth. His Clintonesque umbrage about being accused of having sex with "this woman" lead one to believe there is more to the story that meets the eye.

The biblical narrative portrays Abner as being aware that God had chosen David as the rightful king by this time, so we may be generous and declare Abner's action in going over to David's side as providential, even if externally treasonous. Without Abner's abandoning Ish-bosheth, it is likely that the civil war between Judah and the northern tribes could have continued indefinitely; and the glorious reigns of David and Solomon over the "United Kingdom" might never have occurred.
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