Jamieson, Fausset and Brown on Judges 21:6

6. There is one tribe cut off from Israel this day--that is, in danger of becoming extinct; for, as it appears from Jud 21:7, they had massacred all the women and children of Benjamin, and six hundred men alone survived of the whole tribe. The prospect of such a blank in the catalogue of the twelve tribes, such a gap in the national arrangements, was too painful to contemplate, and immediate measures must be taken to prevent this great catastrophe.