What does Judges 20:3 mean?
Eleven of the twelve tribes of Israel have gathered at the town of Mizpah (Judges 20:1–2), near the Benjaminite city of Gibeah (Judges 19:14). They act in a rare moment of unity and submission to the Lord. They have come in response to a call to action from a Levite man. In a gruesome message, he sent the twelve pieces of his concubine's corpse around the region. She had been raped and murdered by the men of Gibeah (Judges 19:22–30).As noted here, the people of Benjamin were not invited to this assembly. They heard about it, however, as one would expect. The Mizpah mentioned here is barely an hour's walk from the Benjaminite town of Gibeah. According to this passage, Israel has amassed a massive army. Clearly, the other tribes already plan to punish Gibeah. The tribe of Benjamin, for their part, will respond by assembling their own fighting forces.
Given the number of soldiers already present, it's clear the leaders of Israel already determined their response to the atrocity in Gibeah. With the assembly officially begun, the Levite man is now called to speak. Everyone would have already heard some version of his story. Now, though, the leaders of the eleven tribes can hear from him directly, so they can make a formal declaration of their intent.