What does Judges 20:8 mean?

The leaders of eleven of the twelve tribes of Israel have heard the testimony of a Levite (Judges 20:1–7). They had been shocked and compelled to convene when he sent body parts of his dead concubine throughout Israel (Judges 19:29–30). Now they are convinced to take swift action by his statement about what the men of Gibeah did to him and to his concubine.

At this point, the tribes of Israel have already sent a massive number of soldiers. Hearing the Levite's account in person, and meeting as a group, seem to be a formal way of "passing sentence" on Gibeah. The assembled soldiers make a vow, implying they will not rest or return home until justice has been carried out.

The army assembled at Mizpah—a town only a short walk from Gibeah—is only taken from eleven tribes. The twelfth tribe of Israel, Benjamin (Judges 19:14) was excluded from this assembly. Before vengeance can be accomplished, the Benjaminites will have something to say about it. Rather than turning over the guilty, or allowing Israel to simply destroy the town, the tribe of Benjamin will fight back.