שופטים, פרק כ״א, פסוק א׳

The end of a bloody civil war leaves the Israelites facing a tragic reality. Their initial outrage had driven them to a severe decision, one that now places an entire tribe in danger of complete extinction.

Although this solemn oath is mentioned after the descriptions of war and destruction, the declaration actually took place much earlier. The narrative structure points to an action already completed in the past [מלבי״ם]. The promise was made during the impassioned national gathering before the people went to battle [מצודת דוד, ביאור שטיינזלץ]. Specifically, this occurred at the initial assembly before God at Mizpah, right after the Israelites learned of the severe crime in Gibeah and before the fighting began [מצודת דוד, אברבנאל]. Through this decision, the nation intended to boycott the tribe of Benjamin for generations [ביאור שטיינזלץ], decreeing them as outcasts with whom marriage was strictly forbidden [רד״ק]. This harsh punishment was imposed because the tribe had refused to hand over the criminals, effectively backing the wrongdoers and supporting their actions [מצודת דוד, רד״ק, אברבנאל].

The story brings up this prior oath at this specific moment to explain the background of a sudden new crisis. The nation must figure out how to find wives for the six hundred Benjamite survivors without violating their past promise [אברבנאל]. Once the fires of war died down and the Israelites realized that an entire tribe was nearly wiped out, their fierce anger was replaced by a deep sadness over the devastating situation they had created [רלב״ג].