What does Judges 20:12 mean?

Eleven of Israel's twelve tribes have gathered to enact justice. They seek to punish a brutal atrocity (Judges 19:22–27) committed by the men of Gibeah, which belongs to the twelfth tribe, Benjamin (Judges 19:14). Evidence has been seen, and witnesses have been heard. The decision has been made to bring vengeance on the wicked men who brought such evil to Israel. The combined forces of the nation—other than the tribe of Benjamin—are camped near to Gibeah, in one of several towns bearing the name "Mizpah."

Despite amassing troops, the other tribes don't intend to pursue a full-scale war. Before attacking, a delegation is sent to the tribe of Benjamin. The messengers will ask, very reasonably, for the rest of Benjamin's people to participate in punishing the unashamed rapists and murderers of Gibeah. If the Benjaminites agree, all Israel will be united in condemning the horrific acts committed there.