Background:
Abner, the son of Ner, was a prominent military leader and the commander of Saul's army. He was a cousin to King Saul and played a significant role in the political and military affairs of Israel during the transition from Saul's reign to David's kingship. Abner's efforts to unite Israel are primarily documented in the books of 1 Samuel and 2 Samuel.

Abner's Role After Saul's Death:
Following the death of King Saul and his sons at the Battle of Mount Gilboa, Israel faced a leadership crisis. David was anointed king over the tribe of Judah and established his rule in Hebron. Meanwhile, Abner took the initiative to install Ish-bosheth, Saul's surviving son, as king over the remaining tribes of Israel. This act set the stage for a prolonged conflict between the house of Saul and the house of David.

2 Samuel 2:8-9 : "Meanwhile, Abner son of Ner, the commander of Saul’s army, took Saul’s son Ish-bosheth, brought him over to Mahanaim, and made him king over Gilead, Asher, Jezreel, Ephraim, Benjamin, and all Israel."

Conflict and Tension:
The division between the northern tribes under Ish-bosheth and the tribe of Judah under David led to a civil war. Abner, as the commander of Ish-bosheth's forces, engaged in several battles against David's men. The conflict was marked by skirmishes and a notable battle at the pool of Gibeon, where Abner's forces clashed with those of Joab, David's commander.

2 Samuel 2:17 : "The battle that day was very fierce, and Abner and the men of Israel were defeated by the servants of David."

Abner's Defection to David:
Over time, tensions arose between Abner and Ish-bosheth, particularly over a personal dispute involving Rizpah, a concubine of Saul. Feeling disrespected and recognizing David's growing strength, Abner decided to shift his allegiance. He sought to bring the northern tribes under David's rule, thus uniting the kingdom.

2 Samuel 3:9-10 : "May God punish Abner, and ever so severely, if I do not do for David what the LORD has sworn to him: to transfer the kingdom from the house of Saul and establish the throne of David over Israel and Judah, from Dan to Beersheba."

Negotiations with David:
Abner initiated negotiations with David, promising to rally the elders of Israel to support David as king. David welcomed Abner's overtures and hosted him in Hebron, signaling a potential end to the division within the nation.

2 Samuel 3:12 : "Then Abner sent messengers in his place to say to David, 'To whom does the land belong? Make your covenant with me, and surely my hand will be with you to bring all Israel over to you.'"

Abner's Assassination:
Despite the promising negotiations, Abner's efforts were abruptly cut short. Joab, seeking revenge for the death of his brother Asahel, whom Abner had killed in battle, deceitfully lured Abner back to Hebron and murdered him. This act of treachery threatened to derail the unification process.

2 Samuel 3:27 : "Now when Abner returned to Hebron, Joab took him aside into the gateway to speak with him privately, and there he stabbed him in the stomach. So Abner died in revenge for the blood of Asahel, Joab’s brother."

Aftermath:
David publicly mourned Abner's death, distancing himself from Joab's actions and demonstrating his commitment to justice and unity. Abner's assassination, while a setback, did not ultimately prevent the unification of Israel under David's rule. David's lament for Abner and his efforts to honor him helped to solidify his position as the rightful king over a united Israel.

2 Samuel 3:38-39 : "Then the king said to his servants, 'Do you not realize that a great prince has fallen today in Israel? And I am weak this day, though anointed king; and these men, the sons of Zeruiah, are too harsh for me. May the LORD repay the evildoer according to his wickedness.'"