During the war between the house of Saul and the house of David, Abner1 had been strengthening his own position in the house of Saul.
References for 2 Samuel 3:6
Now Saul had had a concubine2 named Rizpah3 daughter of Aiah. And Ish-Bosheth said to Abner, "Why did you sleep with my father's concubine?"
References for 2 Samuel 3:7
Abner was very angry because of what Ish-Bosheth said and he answered, "Am I a dog's head4--on Judah's side? This very day I am loyal to the house of your father Saul and to his family and friends. I haven't handed you over to David. Yet now you accuse me of an offense involving this woman!
References for 2 Samuel 3:8
May God deal with Abner, be it ever so severely, if I do not do for David what the LORD promised5 him on oath
References for 2 Samuel 3:9
and transfer the kingdom from the house of Saul and establish David's throne over Israel and Judah from Dan to Beersheba."6
References for 2 Samuel 3:10
Ish-Bosheth did not dare to say another word to Abner, because he was afraid of him.
Then Abner sent messengers on his behalf to say to David, "Whose land is it? Make an agreement with me, and I will help you bring all Israel over to you."