When Gideon's 300 men blew their horns, what did the LORD cause the Midianites to do?

Answer: Turn their swords on each other

When the 300 horns sounded, "the LORD set all the men in the camp against one another with their swords," and the army fled.

“And when the three hundred rams’ horns sounded, the LORD set all the men in the camp against one another with their swords. The army fled to Beth-shittah toward Zererah as far as the border of Abel-meholah near Tabbath.”— Judges 7:22 (BSB)