What does Judges 21:15 mean?

The people of eleven of Israel's tribes do not want to lose the twelfth tribe, Benjamin. They have pity on its survivors (Judges 20:47–48). This emotion is inspired, so the text says, by God causing a gap in their people. This picture is of a wall with a section missing. The people saw each tribe as part of the structure of the entire nation. Taken out of context, this Scripture might imply God demanded the near extinction of the tribe of Benjamin. Yet the passage is clear this is not the case: the Lord is never quoted as saying Israel should completely wipe out the people of Benjamin. He is never referenced as telling the Israelites to withhold their daughters as brides from Benjamin or to "devote to destruction" any peoples that did not participate in the judgment of Benjamin.

While God didn't intervene in what Israel has done, it does not seem to have been His command that they would create this breach in their own nation. Earlier verses showed the people asking God "why" such a thing had happened, though they themselves were the cause (Judges 21:3). This statement, in one sense, might be an echo of that attitude. It may also be a simple reminder that anything which happens must be allowed by God, even when He does not directly cause it.