In the time of Solomon, Hadad 1-Kings 11:14, according to the Septuagint, "reigned over Edom." It appears by the present passage that the country had been again reduced either by Jehoshaphat, or by an earlier king, and was dependent on the kingdom of Judah, being governed by a "deputy" or viceroy, who, however, was allowed the royal title (compare 2-Kings 3:9, 2-Kings 3:12, 2-Kings 3:26). This government of dependencies by means of subject-kings was the all but universal practice in the East down to the time of Cyrus (the 1-Kings 4:21 note).

There was no king in Edom - It is plain that the compiler of this book lived after the days of Jehoshaphat, in whose time the Edomites revolted; see 2-Kings 8:22. David had conquered the Edomites, and they continued to be governed by deputies, appointed by the kings of Judah, till they recovered their liberty, as above. This note is introduced by the writer to account for Jehoshaphat's building ships at Ezion-geber, which was in the territory of the Edomites, and which showed them to be at that time under the Jewish yoke.

[There was] then no king in Edom: a (b) deputy [was] king.
(b) In the time of this king, Idumea was subject to Judah, and was governed by whom they of Judah appointed.

There was then no king in Edom, a deputy was king. Which had been the case from the times of David, who subdued Edom, and placed garrisons in it, and governors over it, 2-Samuel 8:14 and continued through the reign of Jehoshaphat, unto the times of his son, under whom the Edomites revolted, and set up a king of their own, 2-Kings 8:20, with a view to which this is observed, as also to account for it how Jehoshaphat could build ships in Eziongeber, which was in the land of Edom, of which in the next verse, because the whole country was governed by a viceroy, or deputy, under him.

"There was (then) no (real) king in Edom; a vicegerent was king," i.e., governed the country. This remark is introduced here merely on account of what follows, namely, to show how it was that Jehoshaphat was able to attempt to restore the maritime trade with Ophir. If we observe this connection between the verse before us and what follows, we cannot infer from it, as Ewald does (Gesch. iii. pp. 464 and 474ff.), that the Edomites with Egyptian help had forced from Rehoboam both their liberty and also their right to have a king of their own blood, and had remained in this situation till Jehoshaphat completely subjugated them again. (See the remarks on 1-Kings 11:21-22.) All that can be gathered from 2 Chron 20 is, that the Edomites, in league with the Ammonites and other desert tribes, made an incursion into Judah, and therefore tried to throw off the supremacy of Judah, but did not succeed in their attempt.

A deputy - Sent, and set over them by the kings of Judah, from the time of David, until the days of Jehoram, 2-Chronicles 21:8.