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 20:1THE LEVITE, IN A GENERAL ASSEMBLY, DECLARES HIS WRONG. (Jdg 20:1-7)
all . . . the congregation was gathered as one man--In consequence of the immense sensation the horrid tragedy of Gibeah had produced, a national assembly was convened, at which "the chief of all the people" from all parts of the land, including the eastern tribes, appeared as delegates.
Mizpeh--the place of convention (for there were other Mizpehs), was in a town situated on the confines of Judah and Benjamin (Jos 15:38; Jos 18:26). Assemblies were frequently held there afterwards (Sa1 7:11; Sa1 10:17); and it was but a short distance from Shiloh. The phrase, "unto the Lord," may be taken in its usual sense, as denoting consultation of the oracle. This circumstance, together with the convention being called "the assembly of the people of God," seems to indicate, that amid the excited passions of the nation, those present felt the profound gravity of the occasion and adopted the best means of maintaining a becoming deportment.
jdg 20:3Now the children of Benjamin heard that the children of Israel were gone up to Mizpeh--Some suppose that Benjamin had been passed over, the crime having been perpetrated within the territory of that tribe [Jdg 19:16]; and that, as the concubine's corpse had been divided into twelve pieces [Jdg 19:29] --two had been sent to Manasseh, one respectively to the western and eastern divisions. It is more probable that Benjamin had received a formal summons like the other tribes, but chose to treat it with indifference, or haughty disdain.
jdg 20:4the Levite, the husband of the woman that was slain, answered and said--The injured husband gave a brief and unvarnished recital of the tragic outrage, from which it appears that force was used, which he could not resist. His testimony was doubtless corroborated by those of his servant and the old Ephraimite. There was no need of strong or highly colored description to work upon the feelings of the audience. The facts spoke for themselves and produced one common sentiment of detestation and vengeance.
jdg 20:8THEIR DECREE. (Jdg 20:8-17)
all the people arose as one man--The extraordinary unanimity that prevailed shows, that notwithstanding great disorders had broken out in many parts, the people were sound at the core; and remembering their national covenant with God, they now felt the necessity of wiping out so foul a stain on their character as a people. It was resolved that the inhabitants of Gibeah should be subjected to condign punishment. But the resolutions were conditional. For as the common law of nature and nations requires that an inquiry should be made and satisfaction demanded, before committing an act of hostility or vengeance, messengers were despatched through the whole territory of Benjamin, demanding the immediate surrender or execution of the delinquents. The request was just and reasonable; and by refusing it the Benjamites virtually made themselves a party in the quarrel. It must not be supposed that the people of this tribe were insensible or indifferent to the atrocious character of the crime that had been committed on their soil. But their patriotism or their pride was offended by the hostile demonstration of the other tribes. The passions were inflamed on both sides; but certainly the Benjamites incurred an awful responsibility by the attitude of resistance they assumed.
jdg 20:14the children of Benjamin gathered themselves together out of the cities unto Gibeah--Allowing their valor to be ever so great, nothing but blind passion and unbending obstinacy could have impelled them to take the field against their brethren with such a disparity of numbers.
jdg 20:16left-handed; every one could sling stones at an hair-breadth, and not miss--The sling was one of the earliest weapons used in war. The Hebrew sling was probably similar to that of the Egyptian, consisting of a leather thong, broad in the middle, with a loop at one end, by which it was firmly held with the hand; the other end terminated in a lash, which was let slip when the stone was thrown. Those skilled in the use of it, as the Benjamites were, could hit the mark with unerring certainty. A good sling could carry its full force to the distance of two hundred yards.
jdg 20:18THE ISRAELITES LOSE FORTY THOUSAND. (Jdg 20:18-28)
the children of Israel arose, and went up to the house of God--This consultation at Shiloh was right. But they ought to have done it at the commencement of their proceedings. Instead of this, all their plans were formed, and never doubting, it would seem, that the war was just and inevitable, the only subject of their inquiry related to the precedency of the tribes--a point which it is likely was discussed in the assembly. Had they asked counsel of God sooner, their expedition would have been conducted on a different principle--most probably by reducing the number of fighting men, as in the case of Gideon's army. As it was, the vast number of volunteers formed an excessive and unwieldy force, unfit for strenuous and united action against a small, compact, and well-directed army. A panic ensued, and the confederate tribes, in two successive engagements, sustained great losses. These repeated disasters (notwithstanding their attack on Benjamin had been divinely authorized) overwhelmed them with shame and sorrow. Led to reflection, they became sensible of their guilt in not repressing their national idolatries, as well as in too proudly relying on their superior numbers and the precipitate rashness of this expedition. Having humbled themselves by prayer and fasting, as well as observed the appointed method of expiating their sins, they were assured of acceptance as well as of victory. The presence and services of Phinehas on this occasion help us to ascertain the chronology thus far, that the date of the occurrence must be fixed shortly after the death of Joshua.
jdg 20:29THEY DESTROY ALL THE BENJAMITES, EXCEPT SIX HUNDRED. (Jdg. 20:29-48)
And Israel set liers-in-wait round about Gibeah--A plan was formed of taking that city by stratagem, similar to that employed in the capture of Ai [Jos 8:9].
jdg 20:33Baal-tamar--a palm-grove, where Baal was worshipped. The main army of the confederate tribes was drawn up there.
out of the meadows of Gibeah--Hebrew, "the caves of Gibeah"; a hill in which the ambuscades lay hid.
jdg 20:34there came against Gibeah ten thousand chosen men--This was a third division, different both from the ambuscade and the army, who were fighting at Baal-tamar. The general account stated in Jdg 20:35 is followed by a detailed narrative of the battle, which is continued to the end of the chapter.
jdg 20:45they turned and fled toward the wilderness unto the rock of Rimmon--Many of the fugitives found refuge in the caves of this rocky mountain, which is situated to the northeast of Beth-el. Such places are still sought as secure retreats in times of danger; and until the method of blowing up rocks by gunpowder became known, a few men could in such caves sustain a siege for months.
jdg 20:46all which fell that day of Benjamin were twenty and five thousand men--On comparing this with Jdg 20:35, it will be seen that the loss is stated here in round numbers and is confined only to that of the third day. We must conclude that a thousand had fallen during the two previous engagements, in order to make the aggregate amount given (Jdg 20:15).
jdg 20:48the men of Israel turned again upon the children of Benjamin, and smote them with the edge of the sword--This frightful vengeance, extending from Gibeah to the whole territory of Benjamin, was executed under the impetuous impulse of highly excited passions. But doubtless the Israelites were only the agents of inflicting the righteous retributions of God; and the memory of this terrible crisis, which led almost to the extermination of a whole tribe, was conducive to the future good of the whole nation.