Albert Barnes Commentary Joshua 8:13

"So they set the people, even all the host that was on the north of the city, and their liers-in-wait that were on the west of the city; and Joshua went that night into the midst of the valley." — Joshua 8:13 (ASV)

Joshua went down by night into the valley where he would be seen at daylight by the men of Ai, and was accompanied no doubt by a picked body of troops. The king of Ai, in the morning, would see neither the ambush in his rear, nor the whole of the great host of Israel among the hills away to the north on his left; but supposing, as it appears, that the Israelites before him were a body detached as on the former occasion to assail his city, he sallied out promptly to attack them.