Commentaries:
These two were joining in an alliance to go to war together. They wanted to seek the will of God to see whether it was right for them to do such a thing and to do it together. The king of Israel brought his prophets of Baal before Jehoshaphat, and skeptical of this, Jehoshaphat asked that a true prophet of God be brought. Notice the king of Israel's response regarding God's prophet when Jehoshaphat makes his request: "I do not like him because he does not say things that please me!"
John W. Ritenbaugh
The Sin of Self-Deception