Joshua 8:3 - Ellicott's Commentary On The Whole Bible

And Joshua chose out thirty thousand mighty men. — Some difficulty arises from the fact that thirty thousand men are mentioned as having been sent away with general instructions to form an ambush in the first instance, while five thousand were ultimately posted between Bethel and Ai. Were there two distinct bodies in ambush, or only one? It does not seem possible to answer this question with absolute certainty; but we ought to notice in the first place what the aim of Joshua was. He meant to isolate the town of Ai, taking it in front and flank; but there was another town immediately in the rear, less than two miles off. It was necessary, therefore, to employ a sufficient body of men to close the communications between Bethel and Ai from the first.