‘And you will do to Ai and her king what you did to Jericho and her king. Only its spoil and its cattle you shall take for a prey for yourselves.'
The assurance was that it would be total victory. And the added assurance was that they could now begin to accumulate wealth from the land. YHWH had received His portion at Jericho, a token of what they owed to Him as their overlord. Now they could retain spoils for themselves. Compare on this verse Deuteronomy 2:35; Deuteronomy 3:6 on.
‘ “Set up an ambush for the city, behind it.” So Joshua arose, and all the people of war, to go up to Ai, and Joshua chose out thirty eleph men, the mighty men of valour, and sent them out, and he commanded them, “Look, you shall lie in ambush against the city, behind the city. Do not go very far from the city, but be ready, all of you.” '
YHWH directed tactics. YHWH told him that they were to hide soldiers behind the city, probably making their way there by night. These were to lie in hiding, not far from the ‘city', until after the frontal assault of the ‘city'. Then we are told that Joshua commanded exactly what YHWH had commanded. The way of obedience had also been restored.
“Thirty eleph men.” Ten times more than three eleph sent before. Complacency had been replaced by common sense. This thirty military units was possibly about fifteen hundred men sent to lie in ambush.
These were to go up prior to the main advance (note that ‘arose --- to go up' rather than ‘arose and went up' signifies preparation preparatory to movement). This would take some time. It was an upward climb of over twenty four kilometres (fifteen miles).
Later he would set a further ambush of ‘about five eleph men' to the west of the city (Joshua 8:12). This may have been in order to strengthen the previous force, or in order to give a further prong to the attack. It may have been in case something had prevented the first contingent from taking up its position (no signal may have been spotted). This time he was taking no chances. He was no longer overconfident in their own prowess. And possibly at that stage he had become aware of Bethel looming in the distance.
Some read the text as signifying that the thirty eleph were Israel's total force of which five eleph were put in ambush, but this does readily appear from the text, nor does it tie in with the fact that they had forty eleph Transjordanian troops (Joshua 4:13). We may roughly measure this as indicating that Israel had about fifteen thousand troops, of which fifteen hundred were in the first ambush, and five hundred in the second. (It is to some extent guesswork as we do not really know what an eleph would represent at this time).