When did the Roman Empire fall (your thoughts)?

[Epictetus]

For my own thought, I always believed Rome fell because they lost their identity - that spark that made them special for so many centuries. What was Roman by the 5th century any more? Not much.

But "Rome" was a constantly evolving concept. How does one define it? How much would the Romans under the kings have in common with Scipio, Cicero, Antoninus Pius or Diocletian? Could these diverse people even agree upon a common ideal or goal for the Roman res publica?