Bible Notes Online - 2-Chronicles 21 - ESV
Commentary

v1-3:Jehoshaphat had clearly prepared for his departure. He appointed his sons to serve after him, and provided money and fortified cities for them. This is unselfish.

Although Jehoshaphat had been rebuked by God for an unwise alliance (20.37), he was not discouraged by the rebuke. Instead, he got on to strengthen the kingdom, and prepare his people.

v4-7: After Jehoshaphat had died, Jehoram became king. He killed his brothers, and other leaders in Judah, presumably those who were in the best position to stop him pursuing his rebellion against God. He married Athaliah, daughter of Ahab, and departed from the Lord. We may criticise his alliance with Israel, but Jehoshaphat his father had erred in this way also.

The influence of Athaliah was significant in this period of spiritual declension.

The Lord God remained faithful to His people, and did not destroy the people of Judah.

v8-11: Rebellion against Jehoram; Edom from outside, and Libnah from within. These were direct consequences of Jehoram's disobedience.

By making high places, not only did Jehoram sin, but he encouraged others to continue in sin. No wonder we read that he led Judah astray.

Jehoram also copied Ahab in using chariots. Ahab had died in a chariot (18.33-34).

v12-15: Elijah's ministry was mainly to the northern kingdom, but Judah also failed to follow the ways of the Lord God, so Elijah brought the rebuke to Jehoram. Jehoram, it seems, ignored the letter, and suffered the consequences.

v16-17: More invasions, this time from the Philistines and the Arabs. Their victory seems to have been easy. They reached the kings palace, and removed riches, and took captives.

v18-20: Jehoram died, fulfilling the words of Elijah, and to no one's sorrow.