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 early monarchy of Israel. After Saul's death, Abner initially supported Ish-bosheth, Saul's son, as king over Israel, while David was anointed king over Judah.

Defection to David:
The account of Abner's defection is found in 2 Samuel 3. Tensions arose between Abner and Ish-bosheth, particularly over a personal dispute involving Rizpah, a concubine of Saul. Ish-bosheth accused Abner of taking Rizpah for himself, which was seen as a challenge to the royal house. This accusation angered Abner, who felt his loyalty and contributions were being undermined.

In response to the accusation, Abner decided to shift his allegiance to David. He recognized that David was destined to be king over all Israel, as the Lord had promised. Abner sent messengers to David, proposing a covenant and offering to bring all Israel under David's rule. David agreed to the proposal but demanded the return of his first wife, Michal, Saul's daughter, as a condition for the covenant.

Scriptural Account:
2 Samuel 3:9-10 records Abner's declaration: "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's defection was a significant political move, as he held considerable influence over the tribes of Israel. He communicated with the elders of Israel and the tribe of Benjamin, persuading them to support David. Abner then went to Hebron to meet with David, where they made a feast to celebrate the new alliance.

Aftermath:
Despite the successful negotiation, Abner's defection was short-lived. Joab, the commander of David's army, harbored a personal vendetta against Abner for killing his brother Asahel during a previous battle. Without David's knowledge, Joab deceitfully called Abner back to Hebron and murdered him in revenge.

David mourned Abner's death and distanced himself from the act, declaring his innocence and cursing Joab's house. The death of Abner was a setback for the unification of Israel under David, but it ultimately did not prevent David from becoming king over all Israel.

Significance:
Abner's defection highlights the complex political dynamics and personal rivalries during the transition from Saul's reign to David's kingship. It underscores the fulfillment of God's promise to David and the shifting allegiances that characterized the period of the united monarchy.