Every empire, whether ancient or modern, brings its destruction upon itself. Had it not been barbarians, another culture, or the citizens themselves would have eventually taken over. Sadly, people are corruptable, and although empires and government start with good intentions, with time the corrupt megalomaniacs end up in control. Once that occurs, either the people will rise up, or an adversary will exploit the weakness. The irony, is that once the current regime is deposed, the new regime vows to prevent another. Eventually, they too, become complacent and end up in the same situation as those they replaced.
Viscious, never-ending cycle, but one as humans, we are doomed to repeat.
I'm fascinated by the Roman Empire, partially because I'm of Roman descent, but also because of all the engineering marvels they created, just to be lost until the renassaince. In my humble opinion, (note I said opinion), I see a few countries following in the path of the Romans, the good old US of A included.