Judges 20:34-36

Holman Christian Standard Bible

³⁴ Then 10,000 choice men from all Israel made a frontal assault against Gibeah, and the battle was fierce, but the Benjaminites did not know that disaster was about to strike them. ³⁵ The Lord defeated Benjamin in the presence of Israel, and on that day the Israelites slaughtered 25,100 men of Benjamin; all were armed men. ³⁶ Then the Benjaminites realized they had been defeated.

The men of Israel had retreated before Benjamin, because they were confident in the ambush they had set against Gibeah.