Judges 20:25 - Ellicott's Commentary On The Whole Bible

Destroyed... eighteen thousand men. — This second defeat seems to have been due, like the first, to overweening confidence and carelessness. Thus in two battles the eleven tribes lost 40,000 men — i.e., 13,300 more than the entire Benjamite army, which was only 26,700. Such a hideous massacre can only be accounted for by the supposition that the Benjamite slings did deadly execution from some vantage-ground. Similarly at Crecy “1,200 knights and 30,000 footmen — a number equal to the whole English force — lay dead upon the ground” (Green, 1:419).