Adam Clarke on Joshua 8:17

There was not a man left in Ai or Beth-el - It is very likely that the principal strength of Beth-el had been previously brought into Ai, as the strongest place to make a stand in; Beth-el being but about three miles distant from Ai, and probably not greatly fortified. Therefore Ai contained on this occasion all the men of Beth-el - all the warriors of that city, as well as its own troops and inhabitants. Others think that the Beth-elites, seeing the Israelites fly, sallied out of their city as against a common enemy; but that, finding the men of Ai discomfited, and the city taken, they returned to Beth-el, which Joshua did not think proper to attack at this time. From Jdg 1:24 we find that Beth-el was then a walled city, in the hands of the Canaanites, and was taken by the house of Joseph.