Joshua 8:20

And when the men of Ai looked behind them
On some account or another, perhaps observing that the army of Israel made a full stop and was gazing at the city:

they saw, and, behold, the smoke of the city ascended up to heaven;
from whence they concluded an enemy was there, and had set fire to it:

and they had no power to flee this way or that way;
for if they turned back to their city there was an enemy, how powerful they knew not, possessed of it, and whom they might expect would meet them; and if they pushed forward, there was the whole army of Israel against them, which now turned and faced them, showing no fear of them:

and the people that fled to the wilderness turned back upon the
pursuers;
the people of Israel, that fled by the way of the wilderness, ( Joshua 8:15 ) , turned about, and fell upon the men of Ai that pursued them.