John Gill Commentary 2 Samuel 3:6

"And it came to pass, while there was war between the house of Saul and the house of David, that Abner made himself strong in the house of Saul." — 2 Samuel 3:6 (ASV)

And it came to pass, while there was war between the house of
Saul and the house of David
As long as that continued, as it did until the following quarrel happened between Ishbosheth and Abner:

that Abner made himself strong for the house of Saul ;
or was strongly in the interest of that house, and used his utmost endeavours to support and confirm it.