1 Kings 22:44

And Jehoshaphat made peace with the king of Israel. [One great feature of his reign was this: that the hostility which had lasted, even if it sometimes slumbered, between the two kingdoms for seventy years, from the date of their separation to the time of Asa's death, gave way to peace and even alliance. Judah now recognized the division of the kingdom as an accomplished fact, and no longer treated Israel, even theoretically, as in rebellion. It is probable that the marriage of Jehoram and Athaliah was at once the fruit of, and was intended to cement, this good understanding ( 2 Chronicles 18:1 ). It is hardly likely (Bähr) that the peace was the result of the union of the two families. From the analogy of 2 Chronicles 19:2 ; 2 Chronicles 20:37 ; cf. 1 Kings 16:31 ; 2 Kings 3:14 , we should conclude that the marriage at any rate was ill advised and displeasing to God. Bähr sees in it a step on the part of Jehoshaphat towards realizing the union of the two kingdoms under the supremacy of Judah. He thinks that we cannot otherwise account for this complete change of front.]