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.

The effect to the Midianites would be, that they were surrounded by a mighty host. Their own camp being in darkness, as soon as the confusion of flight began they would mistake friends for foes, and fleers for pursuers. When once fighting had begun by the first casual mistake, the clashing of swords and the shouts of the combatants in the camp, accompanied by the continuous blowing of Gideon's trumpets outside, would make it appear that the whole of the enemy was in the camp. Suspicion of treachery on the part of their allies would also be likely to arise in the minds of Midianites, Amalekites, and Arabs. Compare a similar scene in marginal references.

7:21 They stood - As if they had been torch - bearers to the several companies.