For centuries historians have debated the reasons for the fall of the Roman Empire. The ancient authors pointed to invasions by barbarian armies. But why did the Romans succumb to the invaders when they had been able previously to resist them? Gibbon attributed it to internal weakness: the conversion to Christianity deprived the Romans of their fighting spirit. For many years now scholars have favored an economic explanation: high taxes and heavy economic regulation impoverished the empire. But a recent book by English historian Peter Heather argues that Rome was overcome by illegal immigration. The Germanic tribes did not want to destroy the empire but only to participate in its wealth and the protection it offered its citizens. Though living outside the empire’s borders, the Rhine and the Danube, for several hundred years, from roughly the first to the fifth century, they had been able to trade with Rome, and had grown much wealthier in the process. But wealthier meant more powerful. When the Huns appeared behind them out of the Asian steppes, the Germans became determined to get into the safety of the empire by any means. The Romans, for their part, were not opposed to their desire. On the contrary, Rome from its beginnings was an inclusive society and had always welcomed immigrants. It routinely gave them employment in the Roman army. But immigrants were welcome only under certain conditions. They had to assimilate. They had to disperse throughout the empire’s territory and not insist on remaining in their own groups or maintaining their own culture, but adopt Roman ways of living. Above all, the Romans admitted immigrants only when they could control the process militarily. Any time a tribe of immigrants were permitted to enter, Rome made sure that the empire’s armies outnumbered them by a wide margin, so that there could be no question as to who was in charge, and if the visitors should get obstreperous, they would quickly find themselves quelled.
But in the fifth century the Romans lost control of the immigration process. Armies were sent to the Middle East to counter a hostile, newly invigorated Persia, leaving the West open. The Germanic tribes were allowed in, but once inside the empire they were not assimilated but retained their cultural and political identities, eventually combining to form armies within its borders that the Romans could no longer overcome.
(Peter Heather, The Fall of the Roman Empire, Pan Books, 2005, 158ff.)