It seems to me that the main factors in the fall of the Roman Empire were :-
1) The splitting of the Empire into East and West
2) The mass tribal movements triggered by The Huns
3) The invasion of the North African provinces by the Vandals ( Geiseric )
Rome was sacked in :-
387, by the Gauls
410, by the Goths ( Alaric )
455, by the Vandals ( Geiseric )
These were quick raids and away again, more to get booty for their troops.
These were not cataclysmic events, because the Western empire still had
quite sizeable forces spread around their empire which if they could have
been called back instantly would have been more than enough to deal with
this tribes marauding the Italian mainland.
The Roman empire eventually fell completely about 472AD, quite a significant
time after these sackings.
It seems to me that the greatest blunder of all, was that the Vandals were
allowed to cross from Southern Spain into North Africa.
Of course one can argue that in those days it would have been very difficult
to respond very quickly to something like this ie. Getting intelligence,
notifying Rome, getting a decision, sending some sort of Naval squadron.
However what I can not understand is that since these provinces were the
very lifeblood of Rome ( both in terms of tax, and food production ) that it
was almost criminal negligence that a permanent Naval base was not
established at Gibralter etc., in order to ensure that no undesirables were
allowed to cross into North Africa.
Could this event have been sensibly prevented ? If it could have there is
no reason to believe that the Western Roman Empire would have collapsed
even considering the loss of Gaul, Spain and Britain etc.