After an complete victory at Jericho, Israel suffered defeat at the hands of little Ai because of sin.  In their pride, Israel had attacked without consulting God.  One of the men had taken spoil from Jericho which was devoted for destruction causing God to depart from them.  The story is a picture of the terrible consequences of sin but that also creates a door of hope by causing us to hate sin and seek Jesus to free us from our bondage to it.