I was taught that the main reason the Western empire fell, was that in the end, it relied on barbarian mercenaries.
Compared to the period when Roman citizens made up the army, discipline became lax, morale plummeted and the will to fight became less. Citizens were willing to die for their families, fellow citizens and the glory of Rome. Mercenaries were not.
I think cavalry also had something to do with it. The might of Rome was based on heavy infantry and when the Romans eventually did have cavalry, they were no match for some of the barbarian races that were taught to ride from infancy.