The moral and personal responsibility issues are just an excuse for those who want us to be vigilent and are only tangential issues in the fall of Rome. The military, territorial, and European demographics issues are so immense that everything else is insignificant.
Some people in the US need to keep us on the alert for decay. Hard to fault their motivation, just their sense of history.
If you need a historical analogy, why not use the british empire of France or the Dutch. Or better yet the Soviet Union. We certainly don't want to be like them.
Not really, Considering after the split the Eastern Byzantine Empire lasted another 1100 years which is a pretty good run by any standards.
But I can see the USA breaking in 3.
The Left Coast
The "Red" States
Northeast
Of course The Left Coast will quickly go to the Mexicans the way SW France and Iberia went to the Visigoths after the split and the Northeast will fall apart like the Western empire with NYC becoming a decaying sewer hole just like the city of Rome at the end (though Bloomberg might turn it into that before our split)
The Red states should do fine.