A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
jdg 21:2THE PEOPLE BEWAIL THE DESOLATION OF ISRAEL. (Jdg 21:1-15)
the people came to the house of God, . . . and lifted up their voices, and wept sore--The characteristic fickleness of the Israelites was not long in being displayed; for scarcely had they cooled from the fierceness of their sanguinary vengeance, than they began to relent and rushed to the opposite extreme of self-accusation and grief at the desolation which their impetuous zeal had produced. Their victory saddened and humbled them. Their feelings on the occasion were expressed by a public and solemn service of expiation at the house of God. And yet this extraordinary observance, though it enabled them to find vent for their painful emotions, did not afford them full relief, for they were fettered by the obligation of a religious vow, heightened by the addition of a solemn anathema on every violator of the oath. There is no previous record of this oath; but the purport of it was, that they would treat the perpetrators of this Gibeah atrocity in the same way as the Canaanites, who were doomed to destruction; and the entering into this solemn league was of a piece with the rest of their inconsiderate conduct in this whole affair.
jdg 21:6There is one tribe cut off from Israel this day--that is, in danger of becoming extinct; for, as it appears from Jdg 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.
jdg 21:8there came none to the camp from Jabesh-gilead to the assembly--This city lay within the territory of eastern Manasseh, about fifteen miles east of the Jordan, and was, according to JOSEPHUS, the capital of Gilead. The ban which the assembled tribes had pronounced at Mizpeh seemed to impose on them the necessity of punishing its inhabitants for not joining the crusade against Benjamin; and thus, with a view of repairing the consequences of one rash proceeding, they hurriedly rushed to the perpetration of another, though a smaller tragedy. But it appears (Jdg 21:11) that, besides acting in fulfilment of their oath, the Israelites had the additional object by this raid of supplying wives to the Benjamite remnant. This shows the intemperate fury of the Israelites in the indiscriminate slaughter of the women and children.
jdg 21:16THE ELDERS CONSULT HOW TO FIND WIVES FOR THOSE THAT WERE LEFT. (Jdg 21:16-21)
the elders of the congregation said, How shall we do for wives for them that remain--Though the young women of Jabesh-gilead had been carefully spared, the supply was found inadequate, and some other expedient must be resorted to.
jdg 21:17There must be an inheritance for them that be escaped of Benjamin--As they were the only rightful owners of the territory, provision must be made for transmitting it to their legitimate heirs, and a new act of violence was meditated (Jdg 21:19); the opportunity for which was afforded by the approaching festival--a feast generally supposed to be the feast of tabernacles. This, like the other annual feasts, was held in Shiloh, and its celebration was attended with more social hilarity and holiday rejoicings than the other feasts.
jdg 21:19on the east side of the highway that goeth up from Beth-el to Shechem--The exact site of the place was described evidently for the direction of the Benjamites.
jdg 21:21daughters of Shiloh come out to dance in dances--The dance was anciently a part of the religious observance. It was done on festive occasions, as it is still in the East, not in town, but in the open air, in some adjoining field, the women being by themselves. The young women being alone indulging their light and buoyant spirits, and apprehensive of no danger, facilitated the execution of the scheme of seizing them, which closely resembles the Sabine rape in Roman history. The elders undertook to reconcile the families to the forced abduction of their daughters. And thus the expression of their public sanction to this deed of violence afforded a new evidence of the evils and difficulties into which the unhappy precipitancy of the Israelites in this crisis had involved them.