Background:
Abner, the son of Ner, was the commander of Saul's army and a prominent figure in the early monarchy of Israel. He was a cousin to King Saul and played a significant role in the power dynamics following Saul's death. After Saul's demise, Abner initially supported Ish-bosheth, Saul's son, as king over Israel, while David was anointed king over Judah.

Biblical Account:
The narrative of Abner joining David is primarily found in 2 Samuel 3. The political landscape of Israel was divided after Saul's death, with David ruling over Judah from Hebron and Ish-bosheth ruling over the rest of Israel. Tensions between the house of Saul and the house of David were high, leading to a prolonged conflict.

Abner's Defection:
Abner's decision to join David was precipitated by a falling out with Ish-bosheth. The conflict arose when Ish-bosheth accused Abner of sleeping with Saul's concubine, Rizpah. This accusation angered Abner, who felt his loyalty and efforts were being undermined. In response, Abner decided to shift his allegiance to David, recognizing that God had promised the kingdom to David.

Scriptural Reference:
In 2 Samuel 3:9-10 , Abner declares his intention to support David: "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 sent messengers to David, proposing an alliance and offering to bring all Israel under David's rule. David agreed to meet with Abner, but he stipulated that Michal, Saul's daughter and David's former wife, be returned to him as a condition for the alliance. Abner complied, and Michal was returned to David.

Meeting in Hebron:
Abner visited David in Hebron, where he was warmly received. Abner assured David of his support and promised to rally the elders of Israel to recognize David as king. The meeting was a significant step towards unifying the kingdom under David's rule.

Joab's Reaction:
However, not everyone was pleased with Abner's defection. Joab, David's military commander, harbored a personal vendetta against Abner for killing his brother Asahel in battle. Unbeknownst to David, Joab lured Abner back to Hebron and killed him in revenge.

David's Response:
David was deeply grieved by Abner's murder and publicly mourned his death. He distanced himself from Joab's actions, emphasizing his innocence in the matter. In 2 Samuel 3:38-39 , David lamented, "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."

Significance:
Abner's decision to join David was a pivotal moment in the unification of Israel under David's kingship. It demonstrated the shifting allegiances and the fulfillment of God's promise to David. Despite the tragic outcome of Abner's murder, his defection marked a turning point in the consolidation of David's rule over all Israel.