Albert Barnes Commentary 2 Samuel 2:12

"And Abner the son of Ner, and the servants of Ish-bosheth the son of Saul, went out from Mahanaim to Gibeon." — 2 Samuel 2:12 (ASV)

This expedition to Gibeon may have been for the purpose of shifting his capital to his own tribe of Benjamin and to his family's hometown, "Gibeah of Saul," near Gibeon, with the further purpose of attacking the kingdom of David. The phrase to go out (2 Samuel 2:12–13) is a technical term for going to war (1 Samuel 18:30).