What does Judges 21:1 mean?

Israel acted under God's direction (Judges 20:27–28) to attack and wipe out the city of Gibeah, of the tribe of Benjamin, for the sinful abominations being practiced there (Judges 20:12–13). They killed all Gibeah's inhabitants and burned the city to the ground. But the other eleven tribes did not stop there; the victorious armies then continued to obliterate every remaining person and city in the territory. Scripture gives no indication that this was part of God's command. All but six hundred fighting men are dead; the rest are hiding in an area known as the "rock of Rimmon" (Judges 20:47).

To make matters worse for the tribe of Benjamin, the people of the other eleven tribes took an oath before the fighting started (Judges 21:18). They pledged that none of the other tribes of Israel would give their daughters in marriage to any of the men of Benjamin. The oath seems intended to isolate the evil being practiced in Benjamin from infecting the rest of Israel.

The motive behind making this promise may have been good, but it was not a command given by God. He told the Israelites not to intermarry with Canaanite people, to keep from becoming like them (Deuteronomy 7:1–5). By applying that command to their own people in the tribe of Benjamin—and slaughtering all women in the tribe—Israel may have guaranteed one of the twelve tribes would cease to exist.

Breaking an oath made before the Lord was understood to earn God's wrath (Deuteronomy 23:21–23). The Israelites don't seem to even consider breaking this one. Soon, Scripture will mention another oath made prior to the war which Israel is obligated to honor (Judges 21:5).