And the children of Israel said, Who is there among all the tribes of Israel that came not up with the congregation unto the Lord? For they had made a great oath concerning him that came not up to the Lord to Mizpeh, saying, He shall surely be put to death. (Judges 21:5)
After the battle was all over, the Israelites began an investigation to ascertain whether the entire nation had responded to the summons to take part in the war against Benjamin. When the army first assembled, the tribes had sworn an oath against any segment of the Israelites that refused to support the undertaking. The extreme measures were perhaps necessary to enforce cooperation.