John Calvin Commentary Joshua 8:17

"And there was not a man left in Ai or Beth-el, that went not out after Israel: and they left the city open, and pursued after Israel." — Joshua 8:17 (ASV)

And there was not a man left in Ai, etc. It will be clear from the context that some were captured in the city and slain. Therefore, we must hold that the sally was not made by everyone, and that the old men, women, and many others unfit for war, did not rush out into the fields; the meaning simply is that no garrison was left to defend the city.

The same thing is said of Bethel, and from this we may easily conjecture that Bethel, since it was a small, unimportant town, belonged to another power. The inhabitants, however, being unable to defend their own city, abandoned it and offered their entire force to the king of Ai, to whom they were perhaps tributaries.

It is uncertain whether they went to the king of Ai before the arrival of the Israelites to unite their forces with his in the contest, but it is probable that, since they were unable to resist, they had come by agreement into a fortified and more populous city. They thought that they could not possibly be safe unless they were protected under the shadow of a neighboring city superior to their own.