I voted lead Poisoning and also Byzantium but I don’t think we should overlook the fact that a lot of the Roman Emperors felt no moral or practical obligation towards their citizens and more than a few were straight up bananas.
Climate change? Asteroid? Comet?
Did the people and culture within the Empire change such that the Empire was no longer fit to ‘rule’ the people? And so more and more things stopped being effective; tolerance for setbacks, losses and attacks went down; ability to maintain cohesion was lost. Distant problems were harder to deal with because they were compounded by near-home troubles.
I’ll go with an “all of the above” approach. Every state or socio-politico-economic system has certain advantages and disadvantages. When shocks happen the system might recover, or it might change to a new system. Hit the system with enough shocks and it can’t recover. Rome had recovered from challenge after challenge for hundreds of years, you can’t expect states to last forever.
Yeah, China. China is the closest comparison to Rome, but even more so. Asking why China recovered as a unitary state while Rome never did is missing the point. Most states never recover. Rome recovered and recovered and recovered, until it didn’t. China did. I mean, with a different history of the 20th century we could be talking about a disunited China, with lots of competing and squabbling successor states. Think of Taiwan but much more extensive.
Of course the main difference is that “China proper”–the core of the Chinese state/civilization–is a vast and extensive geographic region with few natural barriers. Contrast with the Roman Empire, which was concentrated not in a vast homogenous heartland but around the Mare Nostrum.
China recovered as a unitary state after the Mongols, but no successor state was able to unify the Mediterranean basin, although Justinian came close. Despite literally a thousand years of conflict the Europeans were never able to conquer North Africa until the modern era.
And of course, unification isn’t the same as continuity. If in 100 years we have a much more unified EU that includes lots of North Africa and the Near East, is that a successor state to the Roman Empire? Not really. So why is modern China considered the successor state to 2nd Century Han China? Aside from sharing the same geography? I guess you’re the successor state if you consider yourself the successor, and evidently modern Chinese people do. In that case Rome didn’t fall until 1453. Or maybe 1806.
But again, Rome had various crazy and/or evil emperors for hundreds of years, and while they certainly caused problems, the Empire seemed to carry on all right.
That’s a testament to the underlying strength of the roman beaurocracy not proof of long term sustainability of the practice of worshipping fruit cakes. That sort of thing takes a toll.
I blame Honorius for being weak-willed, disinterested, possibly bordeline retarded, and unable to take some initiative or make a decision on his own even if you put a gun to his head, instead letting himself be piloted around by up-to-no-good advisors.
And his brother over in Constantinople isn’t much better. Actually, I blame Theodosius for producing such useless offspring.
(Adding “useless emperors”, along with “Persians”, to hypothetical future revised version of poll.)
I agree.
Again, I guess, I agree.
FWIW, my vote in my own poll is “Huns” and “Germans”.
I guess the problem with stating the obvious is that it’s no fun to just state the obvious. Which I should have foreseen as a potential issue for this kind of thread. 
Beyond that: I did set about making a timeline of events in Late Antiquity a while ago, and a couple of things jumped out right away.
One thing: When the Empire properly becomes a Western Empire and an Eastern Empire, in 395 (and, yes, I know the whole division of the Empire thing is a lot more complicated than that, and really goes back to the Third Century Crisis or at least Diocletian, but it still seems fair to say that 395 is when they really start sleeping in separate bedrooms - at least for the sake of argument), the West starts circling the drain pretty much right away. It’s just really quick. Theodosius dies, the moron kids split the Empire, and boom, the WRE’s head is under water. 410, Rome is sacked.
Probably not a coincidence. When allocation of the Empire’s resources becomes a matter of sometimes difficult foreign policy (see Stilicho and the Illyria debacle), and all the money is in the East anyway (with the West only getting unruly barbarians), the West is going to be in trouble.
Another thing: Keeping track of all the different factions, power centers and individuals jockeying for position (Roman and/or barbarian), and their movements on the board, is so complicated that it makes your nose bleed. Which I suppose is what makes the whole thing fun, at least if you’re the kind of guy who enjoys cat-herding as a hobby. But it’s also saying something in its own right, I think.
Actually, tell you what: Sometimes pictures are better than words. I’ll just post a screenshot from the timeline I put together, charting major players and leaders (just *major *ones, never mind the bucketload of minor characters I should have put on there as well) around 400 (and, BTW, it just goes on like that).
By contrast, here’s the timeline around Marcus Aurelius and the Marcomannic Wars. I think it kind of illustrates the point I’m trying to make. These are different times.
Putting all that together: At just the time when the Empire needs unity, focus and strong leadership, the whole political situation instead looks like a gawd-awful clusterfuck. Really bad timing for that, I think.
This is an absolutely key point that always gets lost in this sort of discussion. Everyone wants to figure out why, why WHY did the Roman Empire fall apart? What hidden weakness caused this state to fail?
Neglecting that thousands of other states failed over the millennia. Nobody ever wrings their hands wondering what the secret weakness of Kingdom of Mercia was, that lead to it falling apart.
Rome isn’t unique because it collapsed, it is merely another in a long list of thousands of empires that have collapsed. And this is why when people try to draw parallels between Roman history and, say, modern American history it’s just silly. It’s always 476 for these people, when it isn’t 49 BC with Caesar about to cross the Rubicon, when it isn’t Munich in 1938.
What makes the decline and fall of the Roman empire such a compelling parallel to American history compared to, say, the decline and fall of the Polish-Lithuanian Union, or the Austro-Hungarian Empire, or the Kalmar Union, or even the collapse of the Byzantine Empire?
The Roman empire could easily have exploded much earlier. Suppose Marc Antony had won the battle of Actium. How long could Rome have remained a unitary state under that guy? How likely is it that Rome wipes out Carthage? And so on.
Reading Roman history it often seems like watching a game of Europa Universalis, with Rome getting a human player facing only non-player characters. The Roman Empire could easily have collapsed dozens of times, and it was only saved through heroic efforts. So searching Roman history for the seeds of collapse is a fool’s game. Every country has the seeds of collapse, what makes Rome different is not that it collapsed but that it avoided collapse for so long.
How long is “long term”? Four and a half centuries seems like a pretty decent run.
Plus, the really crazy Emperors (Caligula, Elagabulus) seemed to get themselves murdered pretty quickly, so I don’t think they were really that big of an issue. And if anything, the Later-Empire emperors seemed to have been less likely to be loons, as the Empire became more of an explicit military dictatorship, and the Emperors had to be more or less competent Generals.
For what it’s worth, that’s where I had to stop listening to Mike Duncan’s podcast because there were too many actors for me to keep straight in my head.
The fact that Rome, like the contemporary United States, was a world superpower before its collapse, and the others weren’t.
OK, what about the collapse of the British Empire? Or the collapse of the Spanish Empire? Or the collapse of the Portuguese Empire? Or the collapse of the French Empire? Or the collapse of the Russian Empire?
All these countries were world powers or superpowers. So why Rome? I mean, take a look at the collapse of the British Empire. The Brits are much better off in 2015 after their empire collapsed than they were back in 1937 at its height.
It’s fucking stupid, because there are some pretty major differences between the United States and the Roman Empire. We’re much more like the British Empire than we are like the Roman Empire. Or speaking geographically, much more like China because our heartland is gigantic and homogeneous and lacks significant geographic barriers.
And of course, we conflate our particular form of government with it’s 200+ years of continuity with survival as a nation. France has changed governments dozens of times and has had foreign armies marching across it hundreds of times, but there’s still a France. When they chopped off Louis XVI’s head, that wasn’t the end of France. When the Nazi’s occupied Paris that wasn’t the end of France. When the Algerians pulled down the French flag that wasn’t the end of France.
Or take people fretting about the national debt. What happens when we can’t pay it off? It means the end of the United States! No it doesn’t, it means we take a look at all the people we supposedly owe money to and just rip up those IOUs and tell them to fuck themselves. Economic catastrophe! Yes, yes it will be. Does that mean the end of America? No it doesn’t.
Eventually at some point the United States will cease to exist, because all works of man are impermanent. So what?
And also note that Rome didn’t collapse from world superpower to a bunch of barbarians camped in the ruins overnight. In fact, half of it kept on trucking for another thousand years. Slowly declining, sure. And when the Ottomans eventually captured Constantinople in 1453 the Byzantine Empire hadn’t been a superpower for hundreds of years.
That the tribes that broke down the Western Empire might have been more advanced than what the Romans were used to matters little. What really matters is how stagnant Rome was for so long. While the barbarians were getting better, Rome should have been getting still far greater, at a faster rate.
There was nothing holding Rome back from entering the Industrial Age except for their own small-mindedness about such matters. And Rome with Industrial Age tech could have wiped out everybody.
And people do analyze why the British Empire, etc. (even the Kingdom of Mercia). One thing that does set the Fall of the Western Roman Empire off from most others is the long messy period afterwards. When Mercia fell, it was replaced by something similar, etc. So Rome falling was a far greater event than most other collapses.
Don’t give up on it. 
But, yeah, it’ll take a few go-arounds to get it all straight. And even then, you might be left wondering WTF just happened. Or at least I often am. There’s a lot of trees, but it can be hard to find the forest at times.
One thing that strikes me about the time from 376-476 is that most of the time, the players on the board don’t seem to know what time in history they’re in (that is, what we’re calling the time leading up to the fall of the WRE). They’re running around concerned with their own short-term agendas. Which is, I suppose, how it always is, for everyone, at any time in history. Meanwhile, the WRE sort of goes away, without anyone particularly wanting it to. No one ever *decides *to take down the WRE.
I mean, not even someone like Odoacer, when he kicks out the final Western emperor. At that point, the “emperor” is some kid put on the throne by a random usurpation-happy court official who chased Julius Nepos out of Italy, and the “Western Empire” is already reduced to that guy’s back yard. If you told Odoacer he was ending the Roman Empire, I have a feeling he would just have looked at you funny (with the Byzantines standing over to the side wondering why you’re mistaking them for chopped liver). And, of course, no historians actually tell him that. By the time the WRE goes away, it had already gone away.