Albert Barnes Commentary 2 Kings 8:20

"In his days Edom revolted from under the hand of Judah, and made a king over themselves." — 2 Kings 8:20 (ASV)

Edom, which had been subdued by David (2 Samuel 8:14; 1 Kings 11:15–16) but had apparently revolted from Solomon (1 Kings 11:14), was again subjected to Judah in the reign of Jehoshaphat (2 Kings 3:8–26). The Edomites had, however, retained their native kings and, with them, the spirit of independence. They now rose in revolt and fulfilled the prophecy of Genesis 27:40, remaining from henceforth a separate and independent people (Jeremiah 25:21; Jeremiah 27:3; Amos 1:11, and others). Kings of Edom, who seem to be independent monarchs, are often mentioned in the Assyrian inscriptions.