The Decline and Fall of the Roman Empire is too big a subject for even my considerable ego to take on in a single post. Basically there are four broad theories on why it happened: "general malaise" which holds that the empire was built on unsound foundations and doomed from the start; monocausal which fixes the blame on one reason such as economics or disease; catastrophic collapse, where the fall was caused by a series of contingent events and far from inevitable; and "transformation" which focuses on the continuity between Rome and the barbarian kingdoms. I'm the most aligned with the catastrophic collapse theory.
Western historians definitively assign the fall of the Western Roman Empire to September 4, 476 when the last Western Roman emperor Romulus Augustulus was deposed by Odoacer, the first King of Italy. Think about the life of the average citizen of the empire. The decline of his entire world had been playing out over generations. Once in a while there was a major catastrophe like Adrianople or Alaric's sack of the city of Rome. Other than those, it was a gradual crumbling of Roman institutions. Taxes got a little higher, the currency became a little more debased, the highways became a little less secure, the people a little less devoted to the idea of Rome, the barbarians encroaching a little bit further. By the time the last emperor fell and the empire was formally dissolved, it probably did not come as a surprise to the average citizen to the extent they knew anything of it at all. It would have been more like an exhausted sigh that finally someone had put that horse out of its misery.
Post-apocalyptic novels and science fiction stories often describe the end of the world in monocausal terms. The world of the future is the way it is because of a nuclear war, or a zombie apocalypse, or the superflu, or an extraterrestrial invasion. Sometimes this is accompanied by an assumption that without this one major disaster, mankind would have continued ever on in progress and peaceful expansion. And I have to admit, having the world end in one major disaster can make for exciting reading as it produces tales of survival against impossible odds.
I don't think that's how the world will end here in real life. Our collapse is happening so gradually that some liberals will laugh aloud if you warn them of it. Whig history makes me reach for my gun because it assumes that history is one traceable arc culminating in our wonderful selves. The decline and fall of the United States will be a catastrophic collapse. The Immigration Act of 1965, Roe v. Wade, the 1987 amnesty, the Iraq war, Obamacare... all of these were contingent events that could have and should have gone differently. Instead of being outraged over the descent of cities like Detroit into barbarism, we get outraged over Stephen Colbert's jokes. As an aside, Jerry Seinfeld showed us the correct way to fend off the Volunteer Auxiliary Thought Police. The world was OUTRAGED that Seinfeld's comedy tour was mostly made up of white men. Seinfeld replied that he only cared about being funny, not about satisfying every squabbling tribe's monomania. That incident was quickly forgotten as the nomenklatura moved on to look for those more easily shamed and intimidated.
I'm 33 and I expect the United States to break apart within my lifetime. History will move on like it always has, full of ups and downs.