The first half of the verse offers a change of perspective: we see things through the eyes of the Benjamites. They saw
that they had been outsmarted, and were defeated. This helps the reader to have a measure of empathy with them, at their devasting defeat. Such empathy will in fact emerge again in the final episode of the narrative, as something that the nation as a whole feels and prompts the events that bring the story to a close (Judges 21:2, Judges 21:24).1
36 So the people of Benjamin saw that they were defeated.The men of Israel gave ground to Benjamin, because they trusted the men in ambush whom they had set against Gibeah.