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 20:32
32 And the children of Benjamin said, They are smitten down before us, as at the first. But the children of Israel said, Let us flee, and draw them from the city unto the highways.
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