Not a man was left in Ai or Bethel who did not go out after Israel. They left the city open and pursued Israel.
17. And there was not a man left in Ai, etc It will be clear from the context that some were taken in the city and slain, and therefore we must hold that the sally was not by all universally, and that the old men and women and many others unfit for war, did not rush forth into the fields; the meaning simply is, that...
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
And there was not a man left in Ai or Bethel that went not out after Israel,.... For as these two places were very near to each other, but a mile apart, they were in confederacy, and acted together, and could easily be called to the help of each other when required: though there is a difficulty how the men of Bethel could join those...