Judges 20:34-36

20:34 Ten thousand men, well-trained soldiers from all Israel, then made a frontal assault against Gibeah – the battle was fierce. But the Benjaminites did not realize that disaster was at their doorstep. 20:35 The Lord annihilated Benjamin before Israel; the Israelites struck down that day 25,100 sword-wielding Benjaminites. 20:36 Then the Benjaminites saw they were defeated.

The Israelites retreated before Benjamin, because they had confidence in the men they had hid in ambush outside Gibeah.