What does Judges 20:20 mean?

After much investigation, diplomacy, and inquiring of the Lord, a civil war is about to begin between eleven of the tribes of Israel and the twelfth tribe of Benjamin. The point of the attack by the much larger army of Israel (Judges 20:15–17) is the city of Gibeah. It was in that city where mobs of rapists and murderers attacked innocent travelers (Judges 19:22–28), sparking outrage (Judges 20:11–14).

In both ancient and modern warfare, those defending their home territory have a significant advantage over foreign invaders. Defenders typically know the exact layout of the land, buildings, and roads. They are motivated by a common purpose. They can also set up defensive structures to assist their efforts. In this case, Gibeah is defended by the well-prepared army of the tribe of Benjamin. As battle lines are drawn, few would have suspected how difficult Israel's assault would be.