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