Albert Barnes Commentary 2 Chronicles 20:22

"And when they began to sing and to praise, Jehovah set liers-in-wait against the children of Ammon, Moab, and mount Seir, that were come against Judah; and they were smitten." — 2 Chronicles 20:22 (ASV)

The Lord set ambushments - These who lay in wait have been regarded as angels employed by God to confuse the army and cause its destruction, so that the Moabites and Ammonites first united to destroy the Edomites, and then turned upon each other.