וְהָעָ֥ם נִחָ֖ם לְבִנְיָמִ֑ן כִּֽי־עָשָׂ֧ה יְהוָ֛ה פֶּ֖רֶץ בְּשִׁבְטֵ֥י יִשְׂרָאֵֽל׃
And the people repented them for Benjamin, because that the LORD had made a breach in the tribes of Israel.
Read midrashic interpretation of Judges 21:15: and classical rabbinic exegesis.