Thirty thousand men - comparing Joshua 8:3 and Joshua 8:12 ("five thousand men"), there is probably a mistake in the numbers of this verse, where an early copyist may have written the sign for 30,000 instead of that for 5,000.

Sent them away by night - The selected 5,000 would accordingly post themselves in the main ravine between Ai and Bethel in the night and early morning. The neighhorhood in which Ai was situated is described as "a wild entanglement of hill and valley;" and amidst its recesses the detachment could easily shelter itself from observation until Joshua's other measures were taken.

So Joshua arose, and all the people of war, to go up against Ai,.... As the Lord had commanded him:

and Joshua chose out thirty thousand mighty men of valour; out of all the men of war; these were a select company, picked men, not the whole army, as some have thought, for he was ordered to take all the people of war, as he did:

and sent them away by night; from the main army, that they might pass the city and get behind it undiscovered, for they were sent for an ambush; and of these some were to take the city, and be left in it to burn it, and some to smite the men of Ai, as Abarbinel notes.

Accordingly Joshua set out with all the people of war against Ai, and selected 30,000 brave men, and sent them out in the night, with instructions to station themselves as an ambuscade behind the town, and at no great distance from it. As the distance from Gilgal to Ai was about fifteen miles, and the road runs pretty straight in a north-westerly direction from Jericho through the Wady Faran, the detachment sent forward might easily accomplish the distance in a night, so as to arrive on the western side of Ai before the break of day. They were then to hold themselves in readiness to fight. He (Joshua) himself would approach the town with the people of war that remained with him; and if the inhabitants of Ai should come out against him as they did before, they would flee before them till they had drawn them quite away from their town (Joshua 8:5). This was to be expected; "for they will say, They flee before us, as at the first: and we will flee before them" (Joshua 8:6). When this was done, the warriors were to come forth from their ambush, fall upon the town, and set it on fire (Joshua 8:7, Joshua 8:8). Having been sent away with these instructions, the 30,000 men went into ambush, and posted themselves "between Bethel and Ai, on the west side of Ai" (Joshua 8:9), i.e., according to Strauss, in the Wady es Suweinit, to the north-west of Ai, where it forms almost a perpendicular wall, near to which the ruins of Chai are to be found, though "not near enough to the rocky wady for it to be possible to look down its almost perpendicular wall" (Ritter, Erdk. xvi. p. 528). Joshua remained for the night in the midst of the people, i.e., in the camp of that portion of the army that had gone with him towards Ai; not in Gilgal, as Knobel supposes.

So Joshua arose, and all the people of war, to go up against Ai: and Joshua chose out thirty thousand mighty men of valor, and sent them away by night.

8:3 To go up against Ai - That is, to consider about this expedition; not as if all the people of war did actually go up, which was both unnecessary and burdensome: but it seems to be resolved by Joshua and all the council of war, that the thirty thousand here following should be selected for the enterprize. Either, 1, the thirty thousand now mentioned; or,
2. part of them; namely, such as were to lie in wait; and these were only five thousand men, as is expressed, ver.12.

3. So Joshua . chose out thirty thousand mighty men of valour-Joshua despatched thirty thousand men under cover of night, to station themselves at the place appointed for the ambuscade. Out of this number a detachment of five thousand was sent forward to conceal themselves in the immediate precincts of the town, in order to seize the first opportunity of throwing themselves into it [Jos 8:12].

8:3-22 Observe Joshua's conduct and prudence. Those that would maintain their spiritual conflicts must not love their ease. Probably he went into the valley alone, to pray to God for a blessing, and he did not seek in vain. He never drew back till the work was done. Those that have stretched out their hands against their spiritual enemies, must never draw them back.