Jamieson, Fausset and Brown on 2 Chronicles 21:8-10

8-10. the Edomites revolted--That nation had been made dependent by David, and down to the time of Jehoshaphat was governed by a tributary ruler (1Ki 22:47; 2Ki 3:9). But that king having been slain in an insurrection at home, his successor thought to ingratiate himself with his new subjects by raising the flag of independence [Josephus]. The attempt was defeated in the first instance by Jehoram, who possessed all the military establishments of his father; but being renewed unexpectedly, the Edomites succeeded in completely emancipating their country from the yoke of Judah (Ge 27:40). Libnah, which lay on the southern frontier and towards Edom, followed the example of that country.