What led to the end of the Roman Empire?

This question has been one of the great debates of history for many centuries now. Some thinkers have argued the Empire fell because of social causes, like religious turmoil brought about by the rise of Christianity. Others have argued for a military explanation, that the legions lost their discipline and fighting effectiveness compared to neighboring Germanic 'barbarian' warriors who increasingly pushed into the Empire's borders. Others still have advanced various economic causes, like price inflation triggered by a shortage of silver and gold coins and the common imperial practice of 'debasing' the worth of remaining coins by watering them down with cheaper metals like copper (thereby getting a larger number of coins). Some have suggested the plagues of disease fundamentally crippled the Empire (there were several virulent plagues in the 200s) by killing off millions of people and putting the population on a downward spiral. However, all of these theories have considerable problems and limitations -- not the least of which being that they were all going on centuries before the 400s, when the Western Empire collapsed.

I am personally persuaded that the Roman Empire collapsed due to political causes, chiefly rampant civil war. The other factors listed above probably played a role -- but their major consequences were to exacerbate political tensions in the Empire and make civil war more likely. The Western Roman Empire in the 400s was broken apart by Germanic tribes that settled in and conquered one province after another over time. However, the reason that most of these tribes were able to get into the Empire in the first place was because various emperors recruited them as mercenaries to help them fight against political rivals. As emperors were frequently overthrown by rival military officers, they were less and less likely to entrust multiple high-quality legions to other commanders and instead turned more and more to Germanic mercenaries. But when emperors did not need them as mercenaries any longer -- or could not continue to reward them -- these Germanic warriors would raid or conquer Roman territory instead.

It is important to remember, though, that the Roman Empire only ended in Western Europe. In the Eastern Mediterranean, it continued on for many centuries from the second capital of Constantinople. This Eastern Roman Empire is known to modern historians as the 'Byzantine Empire' (though this was not a phrase people at the time used).

Before anyone can start to answer your question, it is necessary to know what you mean by the Roman Empire, and by its end. Neither is quite as obvious as it might seem.

You have to remember that the Empire was divided into two parts, a Western and an Eastern Empire in 285, by Diocletian. Constantine later moved the capital of the E Empire to thecity variously known as Byzantium, Constantinople and Istanbul. The capital of the W empire was intially Rome, but later moved around before the collapse of the W Empire under pressure from Goths and others in the early 5th century. After this time, however, the E Empire continued to regard itself, quite legitimately, as the continuation of the Roman Epire, and indeed called itself Basilea Romanion, Imperium Romanum or Romania.

Since the E Empire, also called by later historians the Byzantine Empire, lingered on until 1453, there is a thousand years of history depending on what you mean.

For now, let's assume that you mean the W Empire, since that is what people usually seem to mean. There are all sorts of factors which could be blamed for the W Empire's demise, and indeed Gibbon (Decline and Fall of the Roman EMpire, 1776) devoted several volumes (7, if I remember aright) to the subject. The following are leading candidates for blame:

1 A refusal to its own fighting, relying on foreign mercenaries instead

2 An unwillingness to pay these mercenaries the rate for the job. Thy put up with it for a while, but eventually decided to take by force what Rome wouldn't offer willingly.

3 Christianity, which Gibbon certainly thought was partly to blame. For a few glorious years in the 4th century, the emperors decreed that freedom of religion was to be offered throughout the empire. That lasted only until christiniaty became top dog. Thereafter, not only was the old traditional religion quickly banned, but the christians started disputing and fighting among themselves as to precisely which flavour of christianity was to be practiced. Dogmatic disputes about minutiae of doctrine were pursued with absurd dedication and persistence, robbing the empire of its life-blood by distracting it from more important questions. Pretty soon the christians were hunting each other down as 'heretics', and by the end of the fourth century, were being killed as a matter of state policy. This continued to be the case in various parts of the western world well into the 19th century, believe it or not.

As Prof Scott said, this has been an enormous debate for centuries. I largely agree with his view, though I would formulate it differently (e.g. barbarian federate troops were not "mercenaries" in any meaningful sense of the word).

Most ancient empires only lasted 100-200 years. Big empires are very hard to hold together for long on ancient economic and communications systems. The Roman Empire lasted about 500 years in the west, an additional 1000 years (!) in the east, and we should add 250 more if we're going to include the Republic's empire-building era. So the real question isn't why the empire fell - empires fall naturally - but how it avoided falling for so long. But for now I'll deal with the question you actually asked.

The fundamental issue was this: the army had always been the driving force in Roman politics ever since the reforms of Marius. The creation of professional legions meant that troops were more loyal to their immediate commanders than they were to any abstract idea of "Rome," and this meant that the empire was vulnerable to civil wars as the warlords struggled with each other for power. This was what brought the Republic down; this was what caused the Anarchy of 235-284, when the empire went through some two dozen emperors in just fifty years and dozens of would-be usurpers. Constantine and his contemporaries repeated the pattern in 310-324, Julian the Apostate was raised to the purple by his troops in 360 (and only the abrupt death of Constantius avoided civil war), etc.

The pattern repeated again in the fifth century in the western empire, with multiple warlords jockeying for power. Some were ethnic Romans and provincials - Boniface, Aetius, Syagrius, Orestes, etc. Some were ethnic "barbarians," either immigrant federate leaders or simply descended from immigrants - Odoacer, Theoderic, Alaric, Stilicho, etc. The Roman army had always recruited anyone willing to serve, so there were always plenty of these in the military. None of them were trying to destroy the empire, but they were trying to get the best position they could for themselves within the empire.

The thing was that this time, unlike prior civil conflicts, nobody was strong enough to really win. The military was stationed out on the frontiers, on the Rhine and Danube and in Pannonia and Illyria, and the provinces were long since thoroughly Romanized and developed, so there was no way for someone who happened to get control of Italy to impose his will on the others against their will. In 476, one of these generals, Odoacer, killed his rival Orestes and deposed the puppet emperor, Orestes' son Romulus Augustulus. Rather than put a target on his back by making himself emperor, he sent the imperial insignia to the eastern emperor at Constantinople, with the idea of having a new emperor sent to them or else just having the whole empire under a single ruler again. But he wanted to be commander-in-chief of the western military, and negotiations broke down. Odoacer was left ruling Italy and not strong enough to overcome his rivals elsewhere, in the Balkans and Africa and Gaul and Spain, nor were they strong enough to overcome him.

As a result, the various warlords were left to govern the territories they happened to have under their control already. The federates tended to win out over the next generation; as "kings" of immigrant barbarian peoples, they had an alternative way to claim political legitimacy in the absence of an emperor, which their Roman rivals couldn't really do. So the provinces of Roman Europe morphed into the kingdoms of the Middle Ages.

Since the empire was so large, they split it in two-- the eastern and western parts. They two sides had differing cultures, languages I think, and thoughts on how things should be /be done. That's the gist of it, can't remember the details.

And eventually, the eastern fell, I think it was called the Byzantine empire??

it's unable to respond in a short message, we have hundreds of books and multiple theories. But the final blow was the Hun invasion.

And the Eastern Empire (Byzantium) survived 1000 more years, till the fall of Constantinople in 1492

I think Asterix & Oblelix may have had something to do with it

republicans