A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
2 Chronicles 20:1
ch2 20:1JEHOSHAPHAT, INVADED BY THE MOABITES, PROCLAIMS A FAST. (2Ch. 20:1-21)
the children of Moab . . . Ammon, and with them other beside the Ammonites--supposed to be rather the name of a certain people called Mohammonim or Mehunim (Ch2 26:7), who dwelt in Mount Seir--either a branch of the old Edomite race or a separate tribe who were settled there.
2 Chronicles 20:2
ch2 20:2from beyond the sea on this side Syria--Instead of "Syria," some versions read "Edom," and many able critics prefer this reading, both because the nomad tribes here mentioned were far from Syria, and because express mention is made of Mount Seir, that is, Edom. The meaning then is: this confederate horde was composed of the different tribes that inhabited the far distant regions bordering on the northern and eastern coasts of the Red Sea. Their progress was apparently by the southern point of the Dead Sea, as far as En-gedi, which, more anciently, was called Hazezon-tamar (Gen 14:7). This is the uniform route taken by the Arabs in their marauding expeditions at the present day; and in coming round the southern end of the Dead Sea, they can penetrate along the low-lying Ghor far north, without letting their movements be known to the tribes and villages west of the mountain chain [ROBINSON]. Thus, anciently, the invading horde in Jehoshaphat's time had marched as far north as En-gedi, before intelligence of their advance was conveyed to the court. En-gedi is recognized in the modern Ainjidy and is situated at a point of the western shore, nearly equidistant from both extremities of the lake [ROBINSON].
2 Chronicles 20:3
ch2 20:3Jehoshaphat . . . proclaimed a fast throughout all Judah--Alarmed by the intelligence and conscious of his total inability to repel this host of invaders, Jehoshaphat felt his only refuge was at the horns of the altar. He resolved to employ the aid of his God, and, in conformity with this resolution, he summoned all his subjects to observe a solemn fast at the sanctuary. It was customary with the Hebrew kings to proclaim fasts in perilous circumstances, either in a city, a district, or throughout the entire kingdom, according to the greatness of the emergency. On this occasion, it was a universal fast, which extended to infants (Ch2 20:13; see also Joe 2:15-16; Jon 3:7).
2 Chronicles 20:5
ch2 20:5Jehoshaphat stood . . . in the house of the Lord, before the new court--that is, the great or outer court (Ch2 4:9) called the new court, probably from having been at that time enlarged or beautified.
2 Chronicles 20:6
ch2 20:6And said, O Lord God of our fathers--This earnest and impressive prayer embraces every topic and argument which, as king and representative of the chosen people, he could urge. Then it concludes with an earnest appeal to the justice of God to protect those who, without provocation, were attacked and who were unable to defend themselves against overwhelming numbers.
2 Chronicles 20:14
ch2 20:14Then upon Jahaziel . . . came the Spirit of the Lord--This prophet is not elsewhere mentioned, but his claim to the inspiration of a prophetic spirit was verified by the calm and distinct announcement he gave, both of the manner and the completeness of the deliverance he predicted.
2 Chronicles 20:16
ch2 20:16they come up by the cliff of Ziz--This seems to have been nothing else than the present pass which leads northwards, by an ascent from En-gedi to Jerusalem, issuing a little below Tekoa. The wilderness of Jeruel was probably the large flat district adjoining the desert of Tekoa, called El-Husasah, from a wady on its northern side [ROBINSON].
2 Chronicles 20:18
ch2 20:18Jehoshaphat bowed his head . . . and all Judah, &c.--This attitude was expressive of reverence to God and His Word, of confidence in His promise, and thankfulness for so extraordinary a favor.
2 Chronicles 20:19
ch2 20:19the Levites . . . stood up to praise the Lord--doubtless by the king's command. Their anthem was sung with such a joyful acclaim as showed that they universally regarded the victory as already obtained.
2 Chronicles 20:20
ch2 20:20as they went forth, Jehoshaphat stood . . . Hear me, O Judah, and ye inhabitants of Jerusalem--probably in the gate of Jerusalem, the place of general rendezvous; and as the people were on the eve of setting out, he exhorted them to repose implicit trust in the Lord and His prophet, not to be timid or desponding at sight of the enemy, but to remain firm in the confident assurance of a miraculous deliverance, without their striking a single stroke.
2 Chronicles 20:21
ch2 20:21he appointed singers . . . that they should praise . . . as they went out before the army--Having arranged the line of procession, he gave the signal to move forwards. The Levites led the van with their musical instruments; and singing the 136th Psalm, the people went on, not as an army marching against an enemy, but returning in joyful triumph after a victory.
2 Chronicles 20:22
ch2 20:22THE OVERTHROW OF HIS ENEMIES. (Ch2 20:22-30)
when they began to sing and to praise the Lord set ambushments against the children of Ammon, Moab, and Mount Seir--Some think that this was done by angels in human form, whose sudden appearance diffused an uncontrollable panic. Others entertain the more probable opinion that, in the camp of this vast horde, composed of different tribes, jealousies and animosities had sprung up, which led to widespread dissensions and fierce feuds, in which they drew the sword against each other. The consequence was, that as the mutual strife commenced when the Hebrew procession set out from Jerusalem, the work of destruction was completed before Jehoshaphat and his people arrived at the battlefield. Thus easy is it for God to make the wrath of man to praise Him, to confound the counsels of His enemies and employ their own passions in defeating the machinations they have devised for the overthrow of His Church and people.
2 Chronicles 20:24
ch2 20:24when Judah came toward the watchtower in the wilderness--Most probably the conical hill, Jebel Fereidis, or Frank Mountain, from the summit of which they obtained the first view of the scene of slaughter. Jehoshaphat and his people found the field strewed with dead bodies, so that they had not to fight at all, but rather to take possession of an immense booty, the collection of which occupied three days. On the fourth they set out on their return to Jerusalem in the same order and joyful mood as they came. The place where they mustered previous to departure was, from their public thanksgiving service, called, "The Valley of Berachah" ("benediction"), now Wady Bereikut.
2 Chronicles 20:31
ch2 20:31HIS REIGN. (Ch2 20:31-37)
Jehoshaphat reigned over Judah--(See Ch2 24:1).
2 Chronicles 20:32
ch2 20:32walked in the way of Asa his father, and departed not from it--He was more steadfast and consistently religious (compare Ch2 15:18).
2 Chronicles 20:33
ch2 20:33the high places were not taken away--Those on which idolatry was practised were entirely destroyed (Ch2 17:6); but those where the people, notwithstanding the erection of the temple, continued to worship the true God, prudence required to be slowly and gradually abolished, in deference to popular prejudice.
2 Chronicles 20:35
ch2 20:35after this did Jehoshaphat . . . join himself with Ahaziah . . . to make ships--A combined fleet was built at Ezion-geber, the destination of which was to voyage to Tartessus, but it was wrecked. Jehoshaphat's motive for entering into this partnership was to secure a free passage through Israel, for the vessels were to be conveyed across the Isthmus of Suez, and to sail to the west of Europe from one of the ports of Palestine on the Mediterranean. Eliezer, a prophet, denounced this unholy alliance, and foretold, as divine judgment, the total wreck of the whole fleet. The consequence was, that although Jehoshaphat broke off--in obedience to the divine will--his league with Ahaziah, he formed a new scheme of a merchant fleet, and Ahaziah wished to be admitted a partner [Kg1 22:48]. The proposal of the Israelitish king was respectfully declined [Kg1 22:49]. The destination of this new fleet was to Ophir, because the Israelitish seaports were not accessible to him for the Tartessus trade; but the ships, when just off the docks, were wrecked in the rocky creek of Ezion-geber.