John Gill Commentary Judges 21

Now the men of Israel had sworn in Mizpeh
Where they were there convened, before the war began; after they had heard the account the Levite gave of the affair, which brought them thither; and after they had sent messengers to Benjamin to deliver up the men of Gibeah, that had committed the wickedness; and after they perceived that Benjamin did not hearken to their demand, but prepared to make war with them.

Then, as they resolved on the destruction of Gibeah, and of all the cities that sent out men against them, even all the inhabitants of them, men, women, and children, entered into an oath, that they would use those men that remained as Heathens, and not intermarry with them, as follows:

saying, there shall not any of us give his daughter unto Benjamin to
wife ;
seeing those that used the wife of the Levite in such a base manner, and those that protected and defended them, deserved to have no wives.

And the people came to the house of God
Not to the city Bethel, as the Targum, Septuagint, and other versions, but to Shiloh, where were the tabernacle and ark; and this is to be understood of the army after they had utterly destroyed the Benjaminites: hence we read of the camp in Shiloh, (Judges 21:12) , here they came not so much to rejoice, and be glad, and to return thanks for the victory they had at last obtained, as to lament the unhappy case of the tribe of Benjamin, and to have counsel and advice, and consider of ways and means to repair their loss:

and abode there till even before God ;
fasting and praying, instead of feasting and rejoicing:

and lifted up their voices, and wept sore ;
not so much, or at least not only for the 40,000 Israelites that were slain, but for the tribe of Benjamin, in danger of being lost, as follows.

And said, O Lord God of Israel
Jehovah, the only living and true God, the Being of beings, eternal, immutable, omnipotent and omnipresent, the God of all Israel, of the twelve tribes of Israel, their covenant God and Father; who had shown favour to them in such a peculiar and gracious manner, as he had not to other nations, and therefore hoped he would still have a kind regard unto them, and suffer them to expostulate with him in the following manner:

why is this come to pass in Israel ;
expressing, as Abarbinel thinks, a concern for the 40,000 men of Israel which fell in the two first battles; but it manifestly refers to the case in the next words:

that there should be today one tribe lacking in Israel ;
meaning the tribe of Benjamin, which was all destroyed, excepting six hundred men, and these had no wives to propagate the tribe; and therefore, unless some provision could be made for that, it must in a short time be totally extinct; for which they express great concern, it not being their intention when they made the above oath to extirpate them; but such were now the circumstances of things in Providence, that it must perish unless some way could be found to relieve it, and which their oath seemed to preclude; and this threw them into great perplexity.

And it came to pass on the morrow, that the people rose early ,
&c.] The day after their fasting and prayer, and a sense of their present case and circumstances being deeply impressed upon their minds, they rose early in the morning to acts of devotion, and exercises of religion, hoping that being in the way of their duty, the difficulties with which they were perplexed would be removed:

and built there an altar ;
if this place was Bethel, as Kimchi reasons, there Jacob had built an altar; but that in such a course of years might have been demolished: and if it was Shiloh, there was the tabernacle, and so the altar of the Lord there; wherefore this either signifies the repairing of that, being in ruins, which is not likely, since it was but lately used, (Judges 20:26) or the building of a new one, which to do in the tabernacle was not unlawful, especially when the number of sacrifices required it, which it is highly probable was the case now, as it was at the dedication of the temple, (1 Kings 8:64) though the above mentioned writer thinks, that building an altar signifies, as in many places, only seeking the Lord; but the use for which it was built is expressed:

and offered burnt offerings and peace offerings ;
both to atone for the sins they had been guilty of in the prosecution of the war, and to return thanks for victory given, and to implore fresh favours to be bestowed upon them.

And the children of Israel said
One to another, after they had offered their sacrifices, and while they were together in Shiloh:

who is there among all the tribes of Israel, that came not up with the
congregation to the Lord ?
when they were summoned to come to Mizpeh, to consult together about the affair of the Levite's concubine, as appears by what follows:

for they had made a great oath ;
in a very awful and solemn manner, with a curse annexed to it, as that about not giving a wife to Benjamin, (Judges 21:18)

concerning him who did not come up to the Lord to Mizpeh :
not about him who did not go out to battle against Benjamin, nor about every individual who did not come to consult about it; but every city that did not send their proper representatives or quota to assist in that affair:

he shall surely be put to death ;
this was sent along with the summons, in order to quicken their attention to them.