"There was sin in the camp when Israel went up against
Ai, and this was the cause of their defeat. The whole people suffered,
not simply because of Achan’s sin, but because all had sinned.
Habakkuk 2
4 Behold, his soul which is lifted up is not upright in him: but the just shall live by his faith.
--Whether they were blinded by the deceitfulness of sin,
and then 
became exalted in their minds,
--or whether their
self-exaltation led to their sin, is not material; certain it is that
the people had given place to sin, and had become self-confident, which
is in itself sin.
It was not His design that they should have to fight for the possession of the promised inheritance.
We have already seen that numbers and arms had nothing to do with the
taking of Jericho, and that when they depended on their weapons, force
that in ordinary warfare would have been amply sufficient was of no
avail.
Recall also the wonderful deliverance from Egypt, and the overthrow
of the entire army of Pharaoh, without the lifting of a single weapon or
the use of any human power." E.J. Waggoner