2 Kings 8:18
And he walked in the way of the kings of Israel, as did the house of Ahab: for the daughter of Ahab was his wife: and he did evil in the sight of the LORD. And he walked in the way of the kings of Israel, as did the house of Ahab: for the daughter of Ahab was his wife: and he did evil in the sight of the LORD.
The verse belongs to the record of Judah's kings and identifies Jehoram, Joram the son of Jehoshaphat, king of Judah, as a bad king who did evil in the sight of the Lord by walking in the way, practices, and example of the kings of Israel, as did the house of Ahab. He took that evil course because the daughter of Ahab was his wife, and his marriage linked him to Ahab's family and its corrupting influence. The daughter of Ahab who became his wife and queen in Judah was the wicked Athaliah, daughter of the idolatrous parents Ahab and Jezebel, herself idolatrous and a worshiper of Baal.
The marriage had been arranged and permitted by Jehoshaphat, though he was in the main a good and pious king, and it was the worst of his sinful alliances. He permitted his son to marry her in order to strengthen his kingdom by alliance with Ahab, being yoked together with unbelievers. It was a selfish, near-sighted parental scheme for his child's welfare, especially for financial gain.
By that marriage Judah was joined more closely with Israel and tied to the house of Omri. Through it the spirit of Jezebel through her daughter, the curse of Baal-worship, Samaria's idolatries into Jerusalem, all the corruptions of the house of Israel, and Ahab's family influence and wickedness were carried into and infected the whole kingdom of Judah. In this way Jezebel's evil purpose continued through Ahab's descendants to harm Judah as well as Israel, and that alliance brought lasting evil to the kingdom.
Through that marriage Athaliah wielded a controlling influence during the short reigns of her husband Joram and her son Ahaziah, and afterward became the absolute ruler of Judah herself. By that one mistake Judah was destined to be governed sixteen years by wicked rulers. Through his wife's evil influence Jehoram followed the idolatrous course of Israel's kings and did evil, becoming so despotic and cruel that he killed his own brothers with the sword and also many of the princes of Israel, so that one man's selfish blunder caused a whole nation to mourn.