Why couldn't the Romans?
The Late Empire period was not the Rome we see dramatized in movies. The Roman Senate was a city council with little power. The Eastern Empire was prospering, while the West was nearly bancrupt and relations between the two became less symbiotic and more antagonistic. The provinces were completely autonomous nations- the central government in Rome was seen as distant tax collector and the ecomony was becoming more and more fuedal, as generals became warlords with kingdoms and citizens became serfs. Taxes were high, the currency became inflated with less gold in the coins. People bartered goods. The cities suffered from plague. Roads went unrepaired. Border towns became dangerous. People began to look inward.The old virtues had been replaced with a new ideal. Few people were concerned with the influx of barbarians which set up their own kingdoms within the Empire, peace was preferable than the sacrifice of blood for country and most soldiers were barbarians themselves. Few people mourned the death of the Superpower and life continued for many people as if nothing had really changed. By the time Rome was sacked in 453 AD it was mostly a symbolic act- like our own LA riots - the damage was minor. Rome had become not a place, not a nation, but an idea- and soon a memory.