Adam Clarke on Judges 7:21

They stood every man in his place - Each of the three companies kept its station, and continued to sound their trumpets. The Midianites seeing this, and believing that they were the trumpets of a numerous army which had then penetrated their camp, were thrown instantly into confusion; and supposing that their enemies were in the midst of them, they turned their swords against every man they met, while at the same time they endeavored to escape for their lives. No stratagem was ever better imagined, better executed, or more completely successful.