Albert Barnes Commentary 2 Chronicles 19:2

"And Jehu the son of Hanani the seer went out to meet him, and said to king Jehoshaphat, Shouldest thou help the wicked, and love them that hate Jehovah? for this thing wrath is upon thee from before Jehovah." — 2 Chronicles 19:2 (ASV)

Jehu ... went out to meet him - Compare 2 Chronicles 15:2. The monarch was therefore rebuked at the earliest possible moment and in the most effective way, as he was entering his capital at the head of his returning army. Jehu, 35 years previously, had worked in the northern kingdom and prophesied against Baasha (1 Kings 16:1–7), but had now come to Jerusalem as a prophet and historian (compare 2 Chronicles 20:34).

Shouldest thou help ... - As a matter of mere human policy, the conduct of Jehoshaphat in joining Ahab against the Syrians was not only justifiable but also wise and prudent. And the reasoning upon which such a policy was founded would have been unobjectionable but for one circumstance. Ahab was an idolater and had introduced into his kingdom a false religion of a new and most degraded type. This should have led Jehoshaphat to reject his alliance. Military success could only come from the blessing and protection of Yahweh, which such an alliance, if persisted in, was sure to forfeit.