So Joshua and the whole army set out to attack Ai. Joshua chose 30,000 mighty men of valor and sent them out at night
We have here an account of the taking of Ai by stratagem. The stratagem here used, we are sure, was lawful and good; God himself appointed it, and we have no reason to think but that the like is lawful and good in other wars.
Commenting on Joshua 8:3-22
So Joshua arose, and all the people of war, to go up against Ai,.... As the Lord had commanded him: and Joshua chose out thirty thousand mighty men of valour; out of all the men of war; these were a select company, picked men, not the whole army, as some have thought, for he was ordered to take all the people of war, as he...
So Joshua . . . chose out thirty thousand mighty men of valour--Joshua despatched thirty thousand men under cover of night, to station themselves at the place appointed for the ambuscade. Out of this number a detachment of five thousand was sent forward to conceal themselves in the immediate precincts of the town, in order to seize the first opportunity of throwing themselves into it [Jos 8:12].