What does Judges 21:5 mean?
This verse reveals another oath pledged by the people of Israel before going into battle with Benjamin (Judges 20:12–13; 27–28). The first oath was that none of the eleven tribes would give daughters to marry the men of Benjamin. The other is that any clan or family group which did not participate in the assembly of tribes at Mizpah—a summit meant to bring judgment on Gibeah—would be put to death.The intent of this oath seems to have been to unify Israel. The people intended to present a clear message that they were on the side of the Lord, and against the heinous atrocities happening in the Benjaminite city of Gibeah (Judges 19:22–28). Those who would not stand against such sin, or so the thinking seems to be, were guilty of enabling it. This oath may explain why Israel felt compelled to utterly wipe out all men, women, and children of Benjamin—despite no such command from God (Judges 20:47–48).
Only now does Israel investigate to see which groups, if any, didn't send support for the war. The motive for asking this question in this moment becomes clear as the passage continues. In addition to fulfilling an oath, Israel is looking for some way to provide wives for the surviving men of Benjamin without breaking a different oath (Judges 21:6–7).