Jehoshaphat appointed his son Jehoram coregent the year Jehoshaphat went off to do battle with Ahab at Ramoth-gilead (853 B.C.). For the next five years Jehoram served with his father. In 848 B.C. he began ruling alone and did so for the next eight years (until 841 B.C.). His reign overlapped the reigns of Ahaziah and Jehoram (whom the NASB called Joram from now on) in Israel. It is quite possible that the writing prophet Obadiah ministered and wrote the Bible book that bears his name during Jehoram's reign.54
Rather than following the godly example of his father, Jehoram chose to pursue idolatry and infidelity to Yahweh like his wife Athaliah, the daughter of Ahab and Jezebel. If it had not been for His promise to David (2 Sam. 7:12-15), God would have cut off Jehoram's line for his wickedness (v. 19). Instead he disciplined him and Judah by allowing Edom and Libnah to revolt successfully. Edom had come under Judah's control during Jehoshaphat's administration (2 Chron. 20:1-29; cf. 1 Kings 22:47). Zair is another name for Seir or Edom. Chariots did not save Jehoram from defeat (v. 21). Libnah was a town near the border between Judah and Philistia that seems to have revolted when the Philistines invaded Judah (2 Chron. 21:16-17). Judah became weaker under Jehoram because of his wickedness. The king himself died a painful death (2 Chron. 21:18-19).