2 Chronicles 18:2

Jehoshaphat's alliance with Ahab led him to compromise his loyalty to the Lord (see 2 Chronicles 19:2–3; compare 2 Corinthians 6:14–18). His son Jehoram's marriage (2 Chronicles 21:1) to Ahab's daughter Athaliah (22:2) brought Israel's unfaithfulness into Judah (see 21:5–6, 12–13; 22:1–4).

Here is, I. Jehoshaphat growing greater. It was said before (Ch2 17:5) that he had riches and honour in abundance; and here it is said again that his wealth and honour increased upon him by piety and good management.
II. Not growing wiser, else he would not have joined with Ahab, that degenerate Israelite, who had sold himself to work wickedness. What good could he get by a man that was so bad? What good could he do to a man that was so obstinately wicked - an idolater, a persecutor? With him he joined in affinity, that is, married his son Jehoram to Ahab's daughter Athaliah.
1. This was the worst match that ever was made by any of the house of David. I wonder what Jehoshaphat could promise himself by it. (1.) Perhaps pride made the match, as it does many a one, which speeds accordingly. His religion forbade him to marry his son to a daughter of any of the heathen princes that were about him - Thou shalt not take their daughters to thy sons; and, having riches and honour in abundance, he thought it a disparagement to marry him to a subject. A king's daughter it must be, and therefore Ahab's, little considering that Jezebel was her mother. (2.) Some think he did it in policy, hoping by this expedient to unite the kingdoms in his son, Ahab perhaps flattering him with hopes that he would make him his heir, when he intended no such thing.
2. This match drew Jehoshaphat, (1.) Into an intimate familiarity with Ahab. He paid him a visit at Samaria, and Ahab, proud of the honour which Jehoshaphat did him, gave him a very splendid entertainment, according to the splendour of those times: He killed sheep and oxen for him, plain meat, in abundance, Ch2 18:2. In this Jehoshaphat did not walk so closely as he should have done in the ways of his father David, who hated the congregation of evil-doers and would not sit with the wicked (Psa 26:5), nor desired to eat of their dainties, Psa 141:4. (2.) Into a league with Ahab against the Syrians. Ahab persuaded him to join forces with him in an expedition for the recovery of Ramoth-Gilead, a city in the tribe of Gad, on the other side Jordan. Did not Ahab know that that, and all the other cities of Israel, did of right belong to Jehoshaphat, as heir of the house of David? With what face then could he ask Jehoshaphat to assist him in recovering it for himself, whose title to the crown was usurped and precarious? Yet Jehoshaphat, an easy man, yields to go with him: I am as thou art, Ch2 18:3. Some men's kindnesses are dangerous, as well as their society infectious. The feast Ahab made for Jehoshaphat was designed only to wheedle him into the expedition. The kisses of an enemy are deceitful.

Jehoshaphat had riches and honor - The preceding chapter gives ample proof of this.
Joined affinity with Ahab - Took his daughter Athalia to be wife to his son Joram.

And after certain years,.... Two years, according to the Syriac and Arabic versions, or in the third year after the affinity was contracted, see 1 Kings 22:2,

he went down to Ahab to Samaria; to pay him a visit upon the alliance, civil and matrimonial, contracted between them:

and Ahab killed sheep and oxen for him in abundance, and for the people that he had with him; entertained him and his retinue in a very grand and liberal manner:

and persuaded him to go up with him to Ramothgilead; from hence, to the end of the chapter, it is the same with 1 Kings 22:4.