John Gill 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 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 of Ai, when the ambush lay between them both, (Joshua 8:12); they either went another way, or the ambush purposely let them pass, for fear of a discovery by a skirmish with them, and that Bethel as well as Ai might be cleared of its armed inhabitants, and so fall an easy prey to them as well as Ai.

This must be understood only of men of war; for otherwise there were inhabitants left, as old men, and such as were unfit for war, afterwards slain, (Joshua 8:24Joshua 8:25);

and they left the city open :
they did not stay to shut the gates, nor left porters or any guards about, to take care of, protect, and defend the city:

and pursued after Israel ;
with great eagerness and vehemence, not having the least apprehension of their city being in any danger.