The war remained doubtful with an uncertain issue, but in the third engagement the people of Benjamin were delivered to the people of Israel, and being condemned by the divine judgment [they] paid the penalty for their widely immoral behavior.
Judges 20:36
36 So the children of Benjamin saw that they were smitten: for the men of Israel gave place to the Benjamites, because they trusted unto the liers in wait which they had set beside Gibeah.
Commentaries
Begging the Lord's peace, they returned more boldly to war, and they to whom prayer had given courage and who had entertained much hope were now able to do what they planned. On a pretext of withdrawing their front lines, setting ambushes at night in the rear of the city, where a segment of the enemy was located, they followed as some retired and thus were provided with an opportunity for invading the unprotected city. Fires were quickly set and flared up while raging flames and waves of heat revealed the sight of the taken city.
Source: LETTER 33