Albert Barnes Commentary Judges 20:1

"Then all the children of Israel went out, and the congregation was assembled as one man, from Dan even to Beer-sheba, with the land of Gilead, unto Jehovah at Mizpah." — Judges 20:1 (ASV)

The “congregation” is the technical term for the whole community of the Israelite people. Its occurrence here is an indication of the early date of these transactions.

From Dan to Beer-sheba - We cannot safely infer from this expression that the settlement of Dan, recorded in Judges 18:0, had taken place at this time. It only proves that in the writer’s time, from Dan to Beer-sheba was a proverbial expression for all Israel (compare the marginal reference).

With the land of Gilead - This phrase refers to all the trans-Jordanic tribes. These tribes are mentioned particularly both to show that the whole congregation of the children of Israel, in its widest meaning, took part in the council, and also because of Jabesh-Gilead (Judges 21:8, Judges 21:10).

Unto the Lord in Mizpeh - The phrase “unto the Lord” implies the presence of the tabernacle (Judges 11:11 note).

Mizpeh in Benjamin (Joshua 18:26), from its connection with Bethel and Ramah, is probably the one meant here. It is the same Mizpeh that appears as a place of national assembly in 1 Samuel 7:5, 1 Samuel 10:17, and 2 Kings 25:23–25.

This Mizpeh must have been near Shiloh and Gibeah, and in the north of Benjamin. The Benjamites were duly summoned with the other tribes, so their absence was stubbornly disobedient (Judges 20:3).