John Gill Commentary 2 Samuel 2:27

"And Joab said, As God liveth, if thou hadst not spoken, surely then in the morning the people had gone away, nor followed every one his brother." — 2 Samuel 2:27 (ASV)

And Joab said, as God lives
Which was the form of an oath, swearing by the living God:

unless you had spoken ;
that is, these words in (2 Samuel 2:14) ; "let the young men arise and play", that he had not given the challenge to fight:

surely then in the morning the people had gone up everyone from
following his brother ;
they would have gone away and never fought at all; they were not desirous of shedding their blood, and following after them to slay them: thus he lays the blame upon Abner, and makes him to be the cause and beginner of the war.

Some render the particle by "if", and give the sense, that if he had spoken what he last did sooner, the people would long before this time have desisted from pursuing them; for it was not from a thirst after their blood, and a desire to luke vengeance on them, that they pursued them, but to bring them to submission, and lay down their arms; for they could not in honour retreat until they desired it; but the former sense seems best, and is the general sense of the Jewish commentators.