What does Judges 20:11 mean?

In most military campaigns described in Judges, a few tribes of Israel joined forces to go to war against an oppressive enemy. The tribes settled into their territories in other parts of the country are often not even mentioned. The sense of the book is that Israel has not stood together as a unified people for quite some time. They lack a king, literally speaking (Judges 17:6; 18:1; 19:1; 21:25), but also lack cohesion as a people.

Now, though, nearly all the people of God's nation have united in extraordinary agreement (Judges 20:1–10). Unfortunately, the occasion bringing them together is an instance of nearly unbelievable cruelty and depravity (Judges 19:22–30). The desire to purge such evil from God's chosen nation is good, but it will come at a cost. The tribe which owns the guilty city, Gibeah (Judges 19:14) will refuse to cooperate (Judges 20:13. The situation will quickly spiral into a bloody civil war.