Ver. 3-9. So Joshua arose, and all the people of war, &c.— Joshua first detached from his army thirty thousand men, and charged them to go in the night-time and lie in ambush on the west side of Ai, at a convenient distance; while, on his part, by break of day, he advanced with all his troops, and appeared before the town, in order to draw out from thence the garrison, which, doubtless, had no idea that the whole army followed. He then informed the officers who commanded the detachment, that, in this case, he would flee, in order to mislead the king of Ai; that then they were to quit their ambush on the signal which he should give them, (ver. 18.) enter Ai, set fire to some houses, to inform him of their success; and in all things punctually obey these instructions as orders delivered from God.
Joshua 8:3-9
3 So Joshua arose, and all the people of war, to go up against Ai: and Joshua chose out thirty thousand mighty men of valour, and sent them away by night.
4 And he commanded them, saying, Behold, ye shall lie in wait against the city, even behind the city: go not very far from the city, but be ye all ready:
5 And I, and all the people that are with me, will approach unto the city: and it shall come to pass, when they come out against us, as at the first, that we will flee before them,
6 (For they will come out after us) till we have drawna them from the city; for they will say, They flee before us, as at the first: therefore we will flee before them.
7 Then ye shall rise up from the ambush, and seize upon the city: for the LORD your God will deliver it into your hand.
8 And it shall be, when ye have taken the city, that ye shall set the city on fire: according to the commandment of the LORD shall ye do. See, I have commanded you.
9 Joshua therefore sent them forth: and they went to lie in ambush, and abode between Bethel and Ai, on the west side of Ai: but Joshua lodged that night among the people.