Albert Barnes Commentary Joshua 8:1

"And Jehovah said unto Joshua, Fear not, neither be thou dismayed: take all the people of war with thee, and arise, go up to Ai; see, I have given into thy hand the king of Ai, and his people, and his city, and his land;" — Joshua 8:1 (ASV)

God rouses Joshua from his dejection (Joshua 7:6) and bids him march against Ai with the main body. Although Ai was only a small city (compare Joshua 8:25 and Joshua 7:3), the discouragement of the people made it inadvisable to send a mere detachment against it a second time. Moreover, as appears from Joshua 8:17, the people of Ai had help from Bethel and possibly from other places as well.

It was also fitting that all the people should witness with their own eyes the happy consequences of having faithfully put away the sin which had separated them from God.