‘ And the children of Benjamin gathered themselves together out of the cities, to Gibeah, to go out to battle against the children of Israel.'
Recognising that the next move would be for the tribal confederacy to attack Gibeah, the Benjaminites gathered their fighting men together there in order to fight off any attack. They were determined to protect it and defend it against the other tribes. It was their city and no one else had a right to interfere. But this was not only a breach of the covenant, it an act of civil war.
Of course, had they been more conciliatory and agreed to try the men themselves things would have taken a different turn. But now it was prestige that was at stake, and in order to defend that they were prepared to overlook gross sin. So do men behave in their folly. The case was not well thought out. In the end, although they were powerful fighters, they had no hope against such superior numbers. Perhaps they hoped that the tribal confederation would back down, but they had not counted on the effect on the confederate leaders of receiving a part of a woman's torso with its consequent realisation of how great the sin had been against God.