Dredging up an old thread because I just finished Gibbon's The History of the Decline and Fall of the Roman Empire and Pirenne's Mohammed and Charlemagne and then Medieval Cities: Their Origins and the Revival of Trade.
I'm not sure I'm particular compelled as either Gibbon or Pirenne from having a complete answer but I favor Pirenne's explanations that economics and trade are more causative than a loss of civic virtue or the introduction of Christianity.
Originally Posted by Zeno
One view point:
Gibbon offers an explanation for why the Roman Empire fell, a task made difficult by a lack of comprehensive written sources, though he was not the only historian to tackle the subject.
According to Gibbon, the Roman Empire succumbed to barbarian invasions in large part due to the gradual loss of civic virtue among its citizens. They had become weak, outsourcing their duties to defend their Empire to barbarian mercenaries, who then became so numerous and ingrained that they were able to take over the Empire. Romans, he believed, had become effeminate, unwilling to live a tougher, "manly" military lifestyle. In addition, Gibbon argued that Christianity created a belief that a better life existed after death, which fostered an indifference to the present among Roman citizens, thus sapping their desire to sacrifice for the Empire. He also believed its comparative pacifism tended to hamper the traditional Roman martial spirit. Finally, like other Enlightenment thinkers, Gibbon held in contempt the Middle Ages as a priest-ridden, superstitious, dark age. It was not until his own age of reason and rational thought, it was believed, that human history could resume its progress.
Gibbon sees the Praetorian Guard as the primary catalyst of the empire's initial decay and eventual collapse, a seed planted by Augustus at the establishment of the empire. He cites repeated examples of the Praetorian Guard abusing their power with calamitous results, including numerous instances of imperial assassination and incessant demands for increased pay.
It should be noted that the Western Roman Empire "fell" in 476; the administrative capitol had been transferred to Ravenna in 402.
The Eastern Roman Empire (Byzantine Empire) lasted almost 1000 years longer although its last 200 years it was just a small state, finally falling in 1453 to the Ottomans.
Note that IMO wiki is not the best source for history but it is reasonable in basic facts and broad structure and commentary. Use with caution.
With this explanation, where Gibbon really fails to compel a point you note: the Byzantine Empire was just as if not more thoroughly Christianized (the western half of the empire maintained very large pockets of paganism, particularly at the highest strata of society). But it survived and thrived into the 1400s.
"The Cause of the Fall of the Roman Empire" is better asked why did the Western half
of the Roman Empire fail since the eastern half, which considered itself wholly Roman and maintain many of its civic customs, continued for another 1000 years.
But "Christianity" and "loss of civic virtue" remain unsatisfactory because we have something of a control group -- one side of the empire is just as Christian and shares roughly equivalent virtues but doesn't disintegrate for another millenuium.
Peirenne' theory is that, starting with the Crisis of the Third Century -- a full hundred years before the eventual Barbarian conquests of Rome -- the western Europe started to suffer from economic degradation and depopulation. This is almost unfathomable for modern people to consider, that a flourishing civilization up until the 2nd century would start a process of slow but sustained and continued depopulation over 500 years.
But that's precisely what happened.
From that point, all of the consequences -- INCLUDING Christianity -- a messianic religion (we often see messianic religions embraced in times of societal disintegration) -- became almost a fait accompli.
When the western half started experiencing depopulation, the tax base erodes, trade devolves, and Rome is forced to start arming and training Barbarian armies to fight off other Barbarian invaders. Not because Romans were too fat and lazy and overfed, and indulging in Christian dogma that said to consistently turn-the-other-cheek.
Again, this is hard to fathom, but Romans were dying off en masse. They didn't have enough people to defend themselves. For instance, by third and fourth century tax collectors wrote with surprise and alarm that when they went into the country-side and outlying towns -- that year over year farms had been turned into abandoned fields, that towns starting on the outer edges of the empire (e.g., beginning in Britain -- where the empire collapsed first) and then closer and closer to Rome itself were practically abandoned. Towns had fallen apart. Aqueducts and bath-houses were no longer maintained, and were populated with squatters. Thriving productive farms were empty, replaced with people scratching out subsistence livings.
Without the ability tax and extract wealth from the rest of the empire -- since wealth and people had eroded and disappeared -- Romans turned to taxing its elites at a higher and higher rate.
With the degradation of order, urban elites and the higher classes started to flee urban areas to what was then the nascent beginning of feudal estates -- causing a collapse of urban civic order and most importantly education systems, and leading to an increasingly economically depraved and less literate and learned urban population who are eventually forced to flee cities to become tenant farmers or fiefs of estate lords.
From there, the Barbarian conquests are a symptom of an already-collapsed civilization, due to processes of depopulation and the collapse of the western economic order.
The Crisis of the Third Century and the strength of Diocletian and Constantine to maintain the empire through the 4th century are better seen as the last gasps of a dying empire, due to processes started in the 2nd century. Ancient Rome is estimated at one point to have had more than one million people. By the 6th century Rome had less than 30k inhabitants. That's just an incredible loss of people.
What you're THEN left with is a question of how and why did a thriving society find itself increasingly depopulated? It's a simple question, but a critical one -- where did all the people go?
To that end, Pirenne is silent but here we have a good post -- that epidemiology is probably the best answer we have:
Originally Posted by Huskalator
IMO the biggest contributor to the fall of Rome was plague. The empire suffered several plagues that decimated it's population. I think a lot of the problems that Rome suffered during it's decline really stem from a lack of population.
tl;dr summary: Huskalator has IMO, the correct answer.
Rome was, unfortunately, likely a victim of its own success but not in the way people think. It wasn't that they were rich, lazy, and abandoned their collective civic spirit. It's that they were dying out. Literally. As the empire expanded and the tentacles of trade extended into the Middle East and then eventually east Asia, the contact Roman traders initiated brought with it germs and viruses western European populations had yet to be exposed to. Increasing progress and urbanization but without modern sanitation through the 2nd century created led to conditions where plagues and disease could run rampant.
The depopulation of the western empire was very much like what we saw 1000+ years later when Europeans landed in North America, and Native Americans were exposed to the first time to the same germs. Europeans marveled at the relative barren openness of the North American continent and the relative degraded states of Indian civilization, particularly when they had heard legends from natives about large inland Indian cities and civilizations but then often found little trace of them. No doubt had Indians or Asians instead arrived in Europe in Late Antiquity or the Early Middle Ages ("the Dark Ages") they'd have thought much the same and potentially been confused as to what just happened given the legends about the grandeur of the Roman civilization that came before it. But processes caused by invisible things (germs) that play out over decades and centuries are hardest to discern, and the victims certainly would have never appreciated precisely what was happening to them.