What Causes Great Nations to Fall?

In our Wisdom Books (homeschool) we are currently studying about some once great and powerful civilizations that have disappeared.  What happened that caused these civilizations to crumble, fall, and disappear?  The one we are currently studying is the Inca Civilization of South America.  It was discovered high in the Andes Moutains of South America.  It was a very wealthy empire from the 1500's and occupied a territory of more than 2500 miles along the western coast.  They built massive temples and walls out of granite stones,  some which weighed 10-20 tons.  They quarried them down river and moved them 2000 ft up a mountain. The stones were fit together so perfectly that in many places a knife blade could not be fitted between them.  How did they do that with no wheels and no written language? 

They grew incredible crops on cut terraces along the mountain sides with complex irrigation and drainage systems.  There is evidence to believe that at one time they knew and worshipped the True God, Creator God.  However, they also worshipped the sun god and then many gods.  As they turned away from the True God and indulged in pagan worship, their leaders became very corrupt and perverted and the nation followed.  They began to open their lives up to evil spirits and practiced divination and eventually gave their gods all they had, including their children.  They horrendously sacrificed their children at the age of 8-9 by placing them in sun-scorched fields to die.  Most of the culture became addicted to the coca (cocaine) leaves and they changed from a hard-working, wealthy civilization to a sick, perverted, weak people.  The Spanish conquistadores came in with a small army and conquered them.

With this culture being so technologically advanced, where are the Incas today?  How is it possible that a civilization so successful simply vanished, leaving only cold, stone monuments as a mute witness to former greatness?

Civilizations do not just die.  A civilization is destroyed not because of the wickedness of unbelievers but because of the carnality and deception of God's people.  Deut. 28 sets before us the choice of blessing or curses.  If our nation's survival depended on your spirituality, would we survive?