‘ And all the people who were in the city (or Ai) were summoned together to pursue after them, and they pursued after Joshua, and were drawn away from the city. And there was not a man left in Ai or Bethel that did not go out after Israel, and they left the city open and pursued after Israel.'
Joshua was not the only tricky general. The first attack was by the men of Ai, but when the retreat began and the king of Ai realised the strength of the force against him, he summoned the reinforcements, which he had hidden away, composed of the men of Bethel who had been secretly drawn from Bethel, who laid in wait ready for when needed. Thus he now emptied Ai of all its armed men in his eagerness to pursue Israel and dissuade them from trying again, leaving the city wide open.
At this point it is necessary to consider the position again with regard to Ai and Bethel. All the stress was laid on Ai. Yet Ai was only an advance post of Bethel. Why would the emphasis then be on Ai? The answer would seem to be that neither the king of Bethel, nor Bethel itself, were captured. It was the king of Ai who was strung up. It was Ai that was destroyed. Thus in the manner of victors it was Ai that was stressed. They captured their city and executed their king, and incidentally destroyed the army of Bethel at the same time.
The people of Bethel were then no longer a threat and for the time being could be left holed up in their city unable to pose any problems to Israel. It was probably considered that at this stage there were more important things to do than besiege Bethel whose power was broken and who from now on could only act in a defensive capacity for the near future. Thus the stress was on what was visibly achieved and not on what was not achieved.