Judges 20:33-36

20:33 All the men of Israel got up from their places and took their positions at Baal Tamar, while the Israelites hiding in ambush jumped out of their places west of Gibeah. 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.