The fall occurred because of a number of reasons, but one of those reasons which gets far too much play, is one mentioned by Grant, that the west was overrun by hordes of barbarians. While the west did face barbarian incursions, the so called hordes of these barbarians have been steadily revised downwards year after year. So while it used to be believed that the Visigoths entered the empire with more than 100,000 people, many now believe that their numbers were as low as 15 - 20,000 (though some continue to maintain their numbers had to hover around 100,000).

In addition, the Empire of the West generally maintained the upper hand when dealing with these barbarians. When the Empire lost control of a situation, they quickly regained it. Take for example Athaulf's ravages in southern Gaul. He was quickly brought to heel by the Roman forces, and his men starved.

My last point is that the Romans, both in the east and in the west invited these barbarians into the empire for use as soldiers. True, the long borders of the west were difficult to maintain, but Romulus Augustulus was not removed from the throne by a barbarian horde which over ran the borders. Instead, he was removed by Odovakar, who had been in the service of Rome, though admittedly he had rebelled.

In my opinion, the barbarian hordes were a factor, but not the only one. They devastated parts of the east as well, some entereing in the east and then moving west. I tend to think that the western empire fell because the eastern emperors (in a stronger position strategically and financially than the west) tended to either ignore the west, or to install weak emperors in the west and therefore not support the maintenance of the west.