Albert Barnes Commentary 1 Kings 22:2

"And it came to pass in the third year, that Jehoshaphat the king of Judah came down to the king of Israel." — 1 Kings 22:2 (ASV)

This visit indicates a complete change in the relationship that had previously existed between the kingdoms of Israel and Judah. The common danger the two kingdoms faced from the growing power of Syria had likely led them to set aside their differences. Jehoshaphat’s eldest son, Jehoram, was married to Athaliah, the daughter of Ahab. However, this family bond had apparently not yet led to any close intimacy, much less to any joint military expeditions. Jehoshaphat seems to have taken no part in Ahab's former Syrian wars, nor did he join in the great league against the Assyrians (see note on 1 Kings 22:1). His visit on this occasion was likely one of simple friendliness, without any political objective.

Ahab, however, turned the visit to his political advantage. From this time until Ahab’s dynasty was displaced by Jehu, very close relations existed between the two kingdoms (1 Kings 22:49; 2 Kings 3:7; 2 Kings 8:28–29; 2 Chronicles 20:36, etc.).