And there was war between Rehoboam and Jeroboam (s) all [their] days.
(s) That is, all the days of Rehoboam's life.

And there was war between Rehoboam and Jeroboam all their days. For though Rehoboam did not enter into an offensive war, and attack the children of Israel, being dissuaded from it by Shemaiah the prophet in the name of the Lord, yet he might maintain a defensive war; and though there were no pitched battles between them as afterwards, in his son's time, yet there might be skirmishes and bickerings on the borders of their countries.

there was war between Rehoboam and Jeroboam--The former was prohibited from entering on an aggressive war; but as the two kingdoms kept up a jealous rivalry, he might be forced into vigilant measures of defense, and frequent skirmishes would take place on the borders.

Was war - Not an invasive war with potent armies, which was forbidden, 1-Kings 12:24, and not revived 'till Abijam's reign, 2-Chronicles 13:1-3, but a defensive war from those hostilities which by small parties and skirmishes they did to one another.