1 Kings 22:2

Jehoshaphat.. came down] The earlier hostility between Judah and Israel (see 1 Kings 15:16) had by this time given place not only to peace but to friendship, which had been cemented (as appears from 2 Kings 8:18) by a marriage between Jehoshaphat's son Jehoram and Ahab's daughter Athaliah. It is possible that the change in the relations of the two countries had been brought about by success on the part of the northern kingdom, and that Judah had become a vassal of its neighbour: at any rate, both on this occasion and on a later one (2 Kings 3:7.), the king of Judah is found aiding the king of Israel in a war which only promoted the interests of the latter. The cessation of hostilities between the two kingdoms was in many ways a benefit to both; but for Judah the connexion with Israel was attended by serious drawbacks, for besides having to furnish assistance in war, it became infected with the Baal worship introduced by Ahab. Jehoshaphat's alliance with Ahab is explicitly condemned in 2 Chronicles 19:2.