John Gill Commentary 2 Samuel 3:12

"And Abner sent messengers to David on his behalf, saying, Whose is the land? saying [also], Make thy league with me, and, behold, my hand shall be with thee, to bring about all Israel unto thee." — 2 Samuel 3:12 (ASV)

And Abner sent messengers to David on his behalf
On his own account, and not on the account of Ishbosheth, or the people of Israel; but to obtain terms for himself, or in his own name, as representing him and standing in his stead; or secretly, as Kimchi explains it, unknown to Ishbosheth, or the people of Israel. The Targum is, ``out of his place;'' he sent them from the place where he was, from Mahanaim:

saying, whose [is] the land ?
the land of Israel, is it not yours, David? verily it is; to whom does it belong but unto you, to whom the Lord has given it? not to any of Saul's posterity: this he ordered the messengers to say in the first place, in order to ingratiate himself to David, and gain his messengers an audience. The Targum is, ``I swear by him that made the earth;'' so Jarchi says, it is an oath by him whose the earth is, even by the living God, whose is the earth, and the fulness thereof:

saying [also], make your league with me ;
he desired to enter into a covenant of friendship with him, that everything that had passed might be forgiven and forgotten:

and, behold, my hand [shall be] with you, to bring about all Israel
unto you :
he promises on his part to do his utmost to bring all Israel under his government and to obedience to him.