This Is Appendix 55 From The Companion Bible.
The marriage between Jehoram and Athaliah was part of Satan's design to introduce idolatry into Judah, so that Athaliah might do for Judah what her mother Jezebel had done for Israel, and thus secure the same result.
Jehoshaphat began by strengthening himself against Israel (2Chronicles 17:1), but married his son to the daughter (an idolatress) of Ahab, the worst of Israel's kings.
The leaven worked, morally and politically; and was then used by Satan for the destruction of the line by which "the seed of the woman" was to come into the world. See Appendix 23 and Appendix 25.
Jehoram, king of Judah, began by killing off all his brethren (2Chronicles 21:4).
The Arabians came and slew all his sons, save the youngest, Ahaziah (2Chronicles 22:1), called also Joash, Jehoahaz (2Chronicles 21:17), and Azariah (2Chronicles 22:6).
Athaliah slew all the sons of Ahaziah on his death (2Kings 11:1. 2Chronicles 22:10), or thought she did; but the infant Joash was rescued.
Joash (called also Jehoash) was hid for six years, while the faithfulness of Jehovah's word hung upon the Divine preservation of that infant's life. Hence Jehoiada's text (2Chronicles 23:3). See, on the whole subject, Appendix 23. And, note the parallel in the history of England, by James I marrying his son Charles to (an idolatress) Henrietta of France, with similar political results.