An explanation currently in vogue is essentially based on the failure of its economic model: at the height of the Empire, the dominant economic model was based on vast agricultural holdings, owned by large landowners and operating essentially on slavery.

The state itself owned land (the tax authorities) operated in the same way and providing part of the grain (frumentum) used to feed the population of the cities thanks to free distributions of wheat.

Alongside this, the small peasantry was not long in disappearing, unable to compete with the large farms, and increasing the flow of the more or less unemployed urban population.

Roman civilization was in fact very much based on clientelism: a wealthy "boss" thus maintained a crowd of "pickers" who lived on his stipends, and swelled the ranks of his political supporters.

Unfortunately, the more the empire grew in size, the more its needs for slave labor increased, the edict of Caracalla having "naturalized" all the inhabitants of the empire as "Roman citizens", it was necessary for him to seek this slave labor only by carrying out new conquests.

In a way, the empire functioned like a Ponzi scheme: it offered the defeated nations the benefits of Romanity only by seeking to expand further.

however, around the 3rd century, the Roman emperors put an end to the expansion of the empire: Hadrian built a wall in Great Britain, Trajan put an end to the wars against the Parthians...

This decision seemed wise at first sight: it It was becoming more and more difficult to maintain troops on such a long frontier and to send legions to war ever further. But the unforeseen consequence was a drying up of the flow of slaves, which put in difficulty the large agricultural exploitations and involved a fall in their output. The decrease in income led to a decrease in state tax revenues: grain shortages and budget reductions which led to social instability and a demographic decline.

Less state revenue also equals less sesterces to pay troops stationed at the frontiers, especially since, in the absence of new conquests, soldiers could no longer "pay on the beast" through the practice of a fruitful pounding. Hence the dissatisfaction of the legions, who began to want to get involved in politics.

Indeed, the imperial system founded by Augustus was not a hereditary monarchy by divine right as in the Middle Ages: in fact Augustus had pretended to respect the legal bases of the ancient Roman republic: the emperor was not (on the paper at least) than the "princeps", the "first of the citizens (a bit like modern dictators who are "only" president-secretary-general-of-the-party-generalissimo-in-chief for life). the absence of a real defined succession system, each general could cherish the dream of donning the imperial purple if he had troops willing to follow him and march on Rome.

It must first of all be recognized that the Roman Empire did not collapse in the year 476 during the abdication of Romulus Augustule.

Nor did the Roman Empire split in two before. Since the beginning of the empire, the emperors, entitled cæsar augustus, normally associated themselves with a younger emperor, who bore the title cæsar tout court. This arrangement was intended to promote a peaceful succession, since there was no right under which the emperor's successor should choose himself.

In some periods there were two equal emperors, both bearing the title Caesar Augustus. Since the era of Emperor Diocletian, who instituted a tetrarchy at the end of the 3rd century, with four emperors who each ruled a part of the Empire, there were usually at least two emperors sharing the provinces of the Empire, without compromising the unitary character of the Empire. A law promulgated by a Caesar, either at Rome, at Constantinople, at Milan, at Ravenna, at Treves, at Nicomedia, at Sirmium, applied equally, at least in principle, to the whole extent of the empire. Rome lost its privileged legal character at the time of Diocletian, at the end of the 3rd century. The year 476 was not the occasion of any interruption of the Roman administration, nor of Roman law.

The Roman Empire had already been officially bilingual, Latin and Greek, for several centuries. It continued to be so until the 7th century, when the last Latin-speaking territories were definitively invaded, and the army became Greek-speaking.

Romulus Augustus was not even the last emperor to have his seat in Italy. Others later ruled in Ravenna and Milan.

So what happened in the year 476? Well, in the Roman Empire nothing special: A short-lived usurping emperor was deposed like many others, by a general, of Germanic origin like many others, who hastened to submit to the emperor in Constantinople, who granted him the title of king. That is all.

The Roman Empire did not collapse for that. On the contrary, it lasted another millennium until the conquest of Constantinople by the Ottomans in the year 1453. The Byzantine Empire never existed. Its citizens called themselves Romans, their state was the Roman Empire, their sovereign, the Roman Emperor. Eastern Christianity, the Muslim world and the Indians, too, always referred to the Roman Empire as it was.

Even if the subject is still open to debate, it is obvious that the reasons for the collapse of the Roman Empire are multiple and that this collapse took place over the long term.

Lasting economic and monetary crisis from the 3rd century AD. The empire has long been financed by the tribute of its conquests. Once reached its maximum size, these monetary receipts dried up, creating an imbalance in the “economic model” of the Empire.

We also often mention, at the social level, the edict of Caracalla which in 212 granted to all (free!) Romans citizenship. This citizenship, which had previously been an enviable status requiring effort, suddenly became available to everyone.

Finally, of course, the military aspect. Such a large empire required numerous troops to protect its borders (the files ), increasingly threatened by “barbarians” envious of the contributions of Romanism and wishing to benefit from it, while at the same time the Romans were less and less less interested in a military career. As a result Rome turned in ever-increasing proportions to auxiliaries, barbarian troops who were promised a place in the Empire in exchange for their military aid.

These troops, when they were victorious, demanded their due and their place - and land - in the empire creating imbalances in the regions where they were installed.

We thus arrived at a situation where barbarian generals were the only ones able to defend a puppet emperor and an empire losing inexorably its peripheral territories. Ultimately, the last emperor was deposed.

However, this collapse must be put into perspective.

The eastern part of the Empire, which would become the Byzantine Empire, continued to exist for almost a millennium with very significant influence and power for several centuries.

The Romanity did not disappear with the collapse of the Western Empire: the institutions, the trade, the Roman culture continued a long time still because the kingdoms which replaced the empire leaned on this Romanity which they revered.

There are a number of reasons which eventually led to the downfall of the mighty roman empire.  

Rome had tangled with Germanic tribes for centuries, but by the 300s “barbarian” groups like the Goths had encroached beyond the Empire’s borders.

Internal Conflicts i.e. overdependence on slave labor 

On top of that, Even as Rome was under attack from outside forces, it was also crumbling from within thanks to a severe financial crisis. Constant wars and overspending had significantly lightened imperial coffers, and oppressive taxation and inflation had widened the gap between rich and poor.