Rome was, after all, Rome! It had survived, and even flourished, after Hannibal's invasions, numerous internal revolts, etc. So there was no need to take any real precautions. Rome could never fall. Until it did.
I see a lot of that thinking in the West today, America included.
I have a simpler view:
Civilizations fall when they are incapable or refuse to defend themselves.
Incapable because they have destroyed their own economic function.
Refusal when they feel their own civilization is unworthy of defense.
Who does that describe?
/johnny