Ishbosheth, Saul's son, [was] forty years old when he began to
reign over Israel
Being born the same year his father began to reign. (See Gill on 1 Samuel 31:6);
and reigned two years;
which some understand of these, and no more; and whereas David reigned seven years and a half over Judah, before he reigned over all Israel, it is thought by the Jewish chronologer F3 that there was a vacancy in the throne of Israel for the space of five years, and so says Kimchi; which vacancy was either before the reign of Ishbosheth, it being a matter in dispute whether he or Mephibosheth should be set up, or after his death; the tribes of Israel being so long before they acknowledged David their king; or Ishbosheth's reign of two years must be in the middle of David's reign over Judah; but there is no need to suppose either of these, for the text says not that Ishbosheth reigned only two years; but the meaning is, as Ben Gersom observes, that he had reigned two years when the following things happened, and a war began, and not by him but by Abner, and carried on by him; and he being an inactive prince, the rest of his reign was reckoned as no reign, whereas he lived and reigned the same length of time David did over Judah; see ( 2 Samuel 3:1 ) ;
but the house of Judah followed David;
kept close to him as their king, yielding a cheerful obedience to him.
F3 Seder Olam Rabba, c. 13. p. 37.