Parallels to our modern era. Perhaps the EU is the Byzantine section of Pax Americana or visa versa.

http://www.historyguide.org/ancient/lecture14b.html

"The end result for the Romans was that they managed to create their own world and they called it the Roman Empire. And their world view became embodied in a pagan cult. This cult was nothing less than the patriotic worship of Rome itself. And throughout the Empire we find the expression Genius Populi Romani celebrated by all Romans. If anything sustained the Empire, it was the conception of the "Genius of the Roman People." The Romans were taught to believe that the destiny of Rome was the destiny of the world and this became embodied in a civil religion which embraced the genius of the Roman people. This civil religion was a secular, pagan religion, in which all men devoted their energies toward public service to state. It was their duty to serve the state. It was virtuous. These duties consisted of service and responsibility because only through responsible service would one come to know virtue.

Despite the obvious fact that the majority of Roman emperors were scheming, devious, opportunistic, or plainly insane, the world view dominated the social life of the Roman citizen of the Empire. The history of the Empire is dotted with political assassinations, strangulations, emperors playing fiddles while Rome burned, court intrigue and rivalry not to mention a widespread incidence of downright insanity or paranoid schizophrenia. In the end, it is extraordinary that the Roman Empire existed for as long as it did. For Edward Gibbon, author of the Decline and Fall of the Roman Empire (3 vols, 1770s), the decline of Rome was natural and required little explanation: "The decline of Rome was the natural and inevitable effect of immoderate greatness. Prosperity ripened the principle of decay; the cause of the destruction multiplied with the extent of conquest; and, as soon as time or accident and removed the artificial supports, the stupendous fabric yielded to the pressure of its own weight. The story of the ruin is simple and obvious: and instead of inquiring why the Roman Empire was destroyed we should rather be surprised that it has subsisted for so long." [Gibbon, Decline and Fall of the Roman Empire, 2nd ed., vol. 4, ed. by J. B. Bury (London, 1909), pp. 173-174.]

It's a complicated question and has occupied the attention of historians for centuries. One thing can be said with certainty -- although Rome ultimately fell  in A.D. 476, the its decline was a process that had been going on for centuries. This goes back to the comment we've been making all along, that Roman strengths eventually became Roman weaknesses. Another thing which we ought to remember is that the Roman Empire was large, and when we speak of the fall of Rome, we are talking about the western half of the Empire. The eastern half survived as the Byzantine Empire until 1453. Lastly, there is no one explanation that accounts for Rome's decline and fall."