Albert Barnes Commentary 2 Chronicles 17:1

"And Jehoshaphat his son reigned in his stead, and strengthened himself against Israel." — 2 Chronicles 17:1 (ASV)

Jehoshaphat ascended the throne in the fourth year of Ahab (1 Kings 22:41), probably after that monarch had contracted his alliance with the royal family of Sidon and before he was engaged in war with Syria. It was thus not unnatural that Jehoshaphat should begin his reign by strengthening himself against a possible attack from his northern neighbor.