Jamieson, Fausset and Brown on 2 Chronicles 20:22

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.