Charles Ellicott Commentary Judges 20:2

"And the chiefs of all the people, even of all the tribes of Israel, presented themselves in the assembly of the people of God, four hundred thousand footmen that drew sword." — Judges 20:2 (ASV)

The chief. —The Hebrew word is pinnoth, “corner-stones,” as in 1 Samuel 14:38; Isaiah 19:13.

Four hundred thousand. —From this, we learn the interesting fact that in their struggles against the Canaanites the number of the people had been diminished one-third—that is, to a far greater extent than they had been diminished by the wanderings in the wilderness. For at the census in the first year of the wanderings their numbers were (including 35,400 of Benjamin) 603,550 (Numbers 1:46); and in the census in the last year they were 601,730, excluding the Benjamites, who, unlike the other tribes, had increased in numbers, for they were then 45,600 in number.

Footmen. —The Israelites were forbidden to use either chariots or cavalry. (See Notes on Judges 1:19; Judges 4:3.)

That drew sword. —Judges 8:10.