Charles Spurgeon Commentary 2 Chronicles 20:22-23

"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. For the children of Ammon and Moab stood up against the inhabitants of mount Seir, utterly to slay and destroy them: and when they had made an end of the inhabitants of Seir, every one helped to destroy another." — 2 Chronicles 20:22-23 (ASV)

There were three or four nations, and some jealousy or mistrust must have manifested itself, or some mistake had been made, and the motley host divided itself into self-destroying bands. The Israelites had nothing to do but to sing.

Perhaps their very singing was the cause of that disruption among the bands. They could not understand it. They had seen people rush to battle with discordant cries, but these were marching along as if they were coming to a wedding feast, singing hymns and chants. That was a new style of fighting.

So the Moabites and the Ammonites thought that there must be something wrong. "Surely there must be some confederates in the camp," they would say. They suspected each other, as wicked people are quick to do, and so they turned on one another, sparing the Israelites all the trouble of killing them.