In the time of Jehoram, Edom1 rebelled against Judah and set up its own king.
References for 2 Chronicles 21:8
So Jehoram went there with his officers and all his chariots. The Edomites surrounded him and his chariot commanders, but he rose up and broke through by night.
To this day Edom has been in rebellion against Judah. Libnah2 revolted at the same time, because Jehoram had forsaken the LORD, the God of his fathers.
References for 2 Chronicles 21:10
He had also built high places on the hills of Judah and had caused the people of Jerusalem to prostitute themselves and had led Judah astray.
Jehoram received a letter from Elijah3 the prophet, which said: "This is what the LORD, the God of your father4 David, says: 'You have not walked in the ways of your father Jehoshaphat or of Asa5 king of Judah.
References for 2 Chronicles 21:12
But you have walked in the ways of the kings of Israel, and you have led Judah and the people of Jerusalem to prostitute themselves, just as the house of Ahab did.6 You have also murdered your own brothers, members of your father's house, men who were better7 than you.
References for 2 Chronicles 21:13
So now the LORD is about to strike your people, your sons, your wives and everything that is yours, with a heavy blow.
You yourself will be very ill with a lingering disease8 of the bowels, until the disease causes your bowels to come out.' "
References for 2 Chronicles 21:15