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.