Root causes of societal decline and collapse

That video of the Roman Empire's collapse is still ringing in my ears.  It made me think of what made the difference between the Empire's rise and its fall.  What is that difference?  It's hard to put a finger upon it.

The rise of the Empire possibly began with Julius Caesar's crossing the Rubicon.  A willful violation of the law.  Yet the Empire rose with that act.  You could consider it the betrayal of the Republic.  Yet, at the end of the Empire, the treachery did not save the Empire at all.  It was fatal to it.  What's the difference between Caesar and the Caesar's that followed?  It wasn't the mere fact of treachery that made the difference.  Both were present at the beginning and at the end.  Hmm.  Perhaps the betrayal was the harbinger of change.  With the betrayal of the Republic, the Roman form of government changed from Republic to Empire.  With the betrayal of the Empire, the Empire crumbled into the rule of the barbarians.

The rise of Caesar ushered in the Empire, which was a form of government that required the rule of men.  What the Empire required were excellent men.  Caesar was excellent, but the late Empire version of Caesar's were not.  What the Republic needed was the adherence to principles, laws, and traditions.  What the Republic needed was faithful men.  Caesar was not such a man, but he did inspire loyalty to himself.  It must be excellence at something and to something--- a principle as in the Republic, or to the man, as in Julius Caesar.  The late Empire was incapable of excellence of either kind.