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.

The Defection:
The account of Abner's defection to David is primarily found in 2 Samuel 3. Tensions between Abner and Ish-bosheth arose when Ish-bosheth accused Abner of taking Saul's concubine, Rizpah, as his own. This accusation angered Abner, who felt his loyalty and efforts in maintaining Saul's house were being undermined.

In response to the accusation, Abner decided to shift his allegiance to David, recognizing that David was destined to be king over all Israel. Abner's defection was a pivotal moment in the unification of the kingdom under David's rule.

Scriptural Account:
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."

Abner then sent messengers to David, offering to bring all Israel under his rule. David agreed to meet with Abner, but only on the condition that Michal, Saul's daughter and David's former wife, be returned to him. Abner complied, and Michal was brought back to David.

Meeting with David:
Abner met with David in Hebron, where he pledged his support and promised to rally the elders of Israel to recognize David as king. In 2 Samuel 3:21 , Abner said to David, "Let me go at once and assemble all Israel for my lord the king, so that they may make a covenant with you, and you may rule over all that your heart desires." David sent Abner away in peace, signaling a new alliance.

Aftermath:
Despite the peaceful meeting, Abner's defection was not without controversy. 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, an act that David publicly condemned.

David mourned Abner's death and distanced himself from the murder, declaring in 2 Samuel 3:38-39 , "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 defection was a crucial step in the consolidation of David's rule over a united Israel. It demonstrated the shifting allegiances and complex political landscape following Saul's death. Abner's recognition of David's divine appointment as king and his efforts to unite the tribes under David's leadership were instrumental in the establishment of the Davidic dynasty.