Asa’s son Jehoshaphat reigned in his place, and he strengthened himself against Israel.
Here we find concerning Jehoshaphat, I. What a wise man he was. As soon as he came to the crown he strengthened himself against Israel, Ch2 17:1. Ahab, an active warlike prince, had now been three years upon the throne of Israel, the vigour of his beginning falling in with the decay of Asa's conclusion.
Commenting on 2 Chronicles 17:1-9
And Jehoshaphat his son reigned in his stead,.... In the stead of Asa his father: and strengthened himself against Israel; the ten tribes, by fortifying his cities, building castles, raising and keeping a large standing army, as the latter part of this chapter shows, and the next verse.
Jehoshaphat - and strengthened himself against Israel - The kingdoms of Israel and Judah were rivals from the beginning; sometimes one, sometimes the other, prevailed. Asa and Baasha were nearly matched; but, after Baasha's death, Israel was greatly weakened by civil contentions, and Jehoshaphat got the ascendancy. See Kg1 16:16-23.