1 Kings 22:32

And it came to pass, when the captains of the chariots saw Jehoshaphat, that they said, Surely it is the king of Israel. And they turned aside to fight against him: and Jehoshaphat cried out.

Cross-reference

2 Chronicles 18:31 is the parallel account, adding that God helped Jehoshaphat and drew the enemy away.

2 Kings 3:7 depicts Jehoshaphat again allying with a northern king in battle, mirroring the dangerous alliance here.