When I was in grade 11 my history teacher explained all the reasons why the mighty Roman Empire fell.
Mostly those reasons dealt with internal decline, ie corruption, Roman decadence, sapping of martial spirit, even lead pipes.
But according to this excellent book I just read, The Fall of the Roman Empire: A New History of Roman and the Barbarians, the real reasons for the fall were external --- Barbarians and lots of them: Huns, Franks, Germans, Goths.
Anyway, I would recommend this book to anybody interested in classical history (and who isn't?).
It's quite a readable narrative and the author does a good job of modernizing Roman concepts, ie he talks about Roman "spin doctors", who tried to make Imperial disasters sound like great victories.
Some things in politics never change.