This refers probably to an early period in Jehoshaphat’s reign - about his eighth or his ninth year - when he closed the long series of wars between the two kingdoms by a formal peace, perhaps at once cemented by a marriage between Jehoram and Athaliah (1 Kings 22:2 note).
Related Commentaries of 1 Kings 22:44
1 Kings 22:44
44 And Jehoshaphat made peace with the king of Israel.