Theodore Dalrymple briefly mentioned something that Henri Pirenne said, that barbarians made up only five percent of the population of the Roman Empire at the moment of its supposed collapse. Pirenne’s larger point was that Arab expansion led to Europe’s decline:
According to Pirenne the real break in Roman history occurred in the 8th century as a result of Arab expansion. Islamic conquest of the area of today’s south-eastern Turkey, Syria, Palestine, North Africa, Spain and Portugal ruptured economic ties to western Europe, cutting the region off from trade and turning it into a stagnant backwater, with wealth flowing out in the form of raw resources and nothing coming back. This began a steady decline and impoverishment so that, by the time of Charlemagne, western Europe had become almost entirely agrarian at a subsistence level, with no long-distance trade.
In a summary, Pirenne stated that “Without Islam, the Frankish Empire would probably never have existed, and Charlemagne, without Muhammad, would be inconceivable.” That is, he rejected the notion that barbarian invasions in the 4th and 5th centuries caused the collapse of the Roman Empire. Instead, the Muslim conquest of north Africa made the Mediterranean a barrier, cutting western Europe off from the east, enabling the Carolingians, especially Charlemagne, to create a new, distinctly western form of government. Pirenne used statistical data regarding money in support of his thesis. Much of his argument builds upon the disappearance from western Europe of items that had to come from outside. For example, the minting of gold coins north of the Alps stopped after the 7th century, indicating a loss of access to wealthier parts of the world. Papyrus, made only in Egypt, no longer appeared in northern Europe after the 7th century; writing reverted to using animal skins, indicating its economic isolation.
Recent archaeological evidence has shown the Pirenne thesis (no catastrophic fall of Rome) to be B.S. See Ward-Perkins’ The Fall of Rome: And the End of Civilization.
This argument is complete bullshit, as Magus J says. Roman population was declining steadily from 300 AD onwards. Reasons for this, and symptoms of it, are readily evident everywhere in recorded Roman history as well as the archeological evidence. By 800 AD the collapse was centuries-old history.
Islam is evil but lying to ourselves to make it a scapegoat for everything is incredibly idiotic, and nobody with any pretense to intellectual integrity should participate in such a thing.