Joshua 8:3-7

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.

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:

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,

(For they will come out after us) till we have drawn them from the city; for they will say, They flee before us, as at the first: therefore we will flee before them.

Then ye shall rise up from the ambush, and seize upon the city: for the LORD your God will deliver it into your hand.