And when they began to sing and to praise, the LORD set ambushments against the children of Ammon, Moab, and {o} mount Seir, which were come against Judah; and they were smitten.

(o) Meaning, the Idumeans who dwelt in mount Seir.

Wesley's Notes

20:22 To sing - So acceptable are the fervent prayers of God's people to God, and so terrible to their enemies. Ambushments - Or, liers in wait, either the holy angels, who appeared in the shape of men, and possibly put on the appearances and visages of the Moabites or Ammonites, and in that shape slew the rest, who supposing this slaughter to be done by a part of their own army, fell upon them, and so broke forth into mutual slaughters. Or, God raised jealousies and animosities among themselves, which broke forth, first into secret ambushments, which one party laid for another, and then into open hostilities to their utter destruction. So vain are all mens attempts against God, who needs none to destroy his enemies but themselves, and their own mistakes, and passions, which he can, when he pleaseth, arm against them.

King James Translators' Notes

And when...: Heb. And in the time that they, etc

to sing...: Heb. in singing and praise

they were...: or, they smote one another

Jamieson-Fausset-Brown Bible Commentary

2Ch 20:22-30. The Overthrow of His Enemies.

22. 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.

Matthew Henry's Concise Commentary

20:20-30 Jehoshaphat exhorted his troops to firm faith in God. Faith inspires a man with true courage; nor will any thing help more to the establishing of the heart in shaking times, than a firm belief of the power, and mercy, and promise of God. In all our trust in the Lord, and our praises of him, let us especially look at his everlasting mercy to sinners through Jesus Christ. Never was an army so destroyed as that of the enemy. Thus God often makes wicked people destroy one another. And never was a victory celebrated with more solemn thanksgivings.