What does Judges 20:18 mean?
Combined armies from eleven of the twelve tribes of Israel stand together, ready to go to war "as one man." They have gathered to purge evil from Israel in the form of Gibeah's culture of open rape and murder of innocent travelers (Judges 19:22–28). They have the will to win, as well as the numbers; Israel's forces outnumber Benjamin's about fifteen-to-one (Judges 20:14–17).This episode would have been over quickly if the people of Benjamin had agreed to hand over the guilty men from Gibeah. Instead, Benjamin has decided to defend them by amassing their own army. War is inevitable. The tribe of Benjamin has settled into a defensive posture.
Before attacking, the leaders of the eleven tribes do something unusual during this period: they turn to the Lord, asking God for direction. Their specific question is which of the eleven tribes' armies should attack first. They present this question to the Lord at Bethel. This probably means the town of Bethel north of Jerusalem, not far from where they amassed at Mizpah. Some scholars suggest bethel might refer to wherever the ark and sacrifices were located, and that this location moved. Other verses indicated the house of God was at Shiloh (Judges 18:31).
Bethel was a special place for meeting with God. At the very beginning of Israel's history, Abraham built an altar to the Lord there (Genesis 12:8), as did Jacob (Genesis 31:13; 35:7, 15). Now the ark of the covenant was being kept at Bethel, with priests overseeing the worship of God (Judges 20:27–28).
Judges does not describe how this delegation from the tribes of Israel made their inquiry of God. This might have been related to the Urim and Thummim associated with the priesthood (Exodus 28:30; Numbers 27:21). Whatever form was used, the Lord heard and graciously responded: Judah was to go first. This answer from God implies that He approved of Israel's commitment to destroying the sin of Gibeah, even if it meant going to war against their own brothers.