John Gill Commentary Joshua 8:7

"and ye shall rise up from the ambush, and take possession of the city: for Jehovah your God will deliver it into your hand." — Joshua 8:7 (ASV)

Then you shall rise up from the ambush
When they should see a signal made, that in (Joshua 8:18Joshua 8:19); and when their watch, they might set in a proper place, should see Israel flee, and the men of Ai at a distance from their city, pursuing, and give notice of it to the ambush:

and seize upon the city ;
enter into it, and take possession of it, its fortresses, and secure its passes:

for the Lord your God will deliver it into your hand ;
Joshua expresses himself with great confidence, having the promise of God, and relying on it.