This question is one of the most fundamental to Western history and has been debated for centuries. I will briefly discuss each of the prominent proposed causes for the decline of Rome throughout history, however this will necessitate a particularly long, however surprisingly brief, discussion of the history and historiography of the fall of the Western Roman Empire.

The most commonly proposed reasons for the fall of Rome are, decadence, decline in militarism, Christianity, barbarization of the armies, disease, devaluation of currency, inept emperors, moral decay, civil war, the Sassanian Empire, over-expansion, and Fundamental flaws in the imperial system. Most theories as to the decline of Rome are based on some combination of these reasons that led to the fall of the Western Roman Empire.

The timeline of the fall of the Western Roman Empire begins with the Crisis of the Third Century. Starting in AD 234, this was a period of intense social and political turmoil which was predicated by the assassination of Emperor Severus Alexander by his troops. This begins the period of Barracks emperors, individuals who seized power through their control of an army, who often reigned for a very short period of time and often never were seated in Rome. In total, during the whole of the crisis, twenty-two men were elevated to emperor, fourteen of which were barracks emperors. This period was marred by massive economic decline, widespread disease, intense civil wars, and poorly faring external wars leaving the empire in massive decline until the reign of Diocletian.

After the Crisis of the Third century, Constantine the Great moved the capitol of the Roman Empire from Rome to Byzantium, renamed Constantinople, and formalized both the legalized status of Christianity as well as Christian doctrine (Council of Nice). An uprising of Visigoths occurred later in the century further taxing the strained imperial military and treasury as well as the last emperor to reign over the full extent of the East and West Roman empires, Theodosius I.

The fifth century was a succession of humiliations of the Western Roman Empire as well as the city of Rome at the hands of the Goths, sacking Rome twice and destruction of Roman armies until the final destruction of the Roman Empire in AD 476.

The primary monumental figure in the historiography of the fall of the Roman Empire is Edward Gibbon, a historian who published his massive history The History of the Decline and Fall of the Roman Empire in 1781. This text became one of the fundamental texts in Roman historiography and although many of his theses are no longer considered accurate, his text is foundational to modern European understanding of the fall of Rome. His main thesis for the decline of the Western Roman Empire was the lack of civic virtue and decline of militarism amoung the Roman populace, requiring the armies to be filled with substandard and expensive mercenaries, with no strong allegiance to Rome. Gibbon bases this fact on the rise of Christianity and the decline in traditional roman values; the rise of a decadent society with no stomach for war.

When the promise of eternal happiness was proposed to mankind on condition of adopting the faith, and of observing the precepts, of the Gospel, it is no wonder that so advantageous an offer should have been accepted by great numbers of every religion, of every rank, and of every province in the Roman Empire. The ancient Christians were animated by a contempt for their present existence, and by a just confidence of immortality, of which the doubtful and imperfect faith of modern ages cannot give us any adequate notion. In the primitive church the influence of truth was very powerfully strengthened by an opinion which, however it may deserve respect for its usefulness and antiquity, has not been found agreeable to experience. It was universally believed that the end of the world, and the kingdom of heaven, were at hand. The near approach of this wonderful event had been predicted by the apostles; the tradition of it was preserved by their earliest disciples, and those who understood in their literal sense the discourses of Christ himself were obliged to expect the second and glorious coming of the Son of Man in the clouds, before that generation was totally extinguished which had beheld his humble condition upon earth, and which might still be witness of the calamities of the Jews under Vespasian or Hadrian.

This quote demonstrates the primary argument of Gibbon, that the proposed afterlife of Christianity quelled and diminished the Roman values which focused on sacrifice for Rome and made Romans more docile for if they lived a good life according to Christian theology, they would be rewarded with a far better life after their death. Gibbon argues that this is a primary source of decline in Roman military standards as the citizen soldier, the core of the Roman military, was diminished in favor of a peaceful life. The decline in Romans in the military led to the inclusion of ever-more barbarians in the military which lowered the standards of service. Several edicts had been passed during the reign of Septimius Severus which have also been argued to further decline the quality of the armies which were further compounded by their barbarization (Roman soldiers were forbidden to marry until his reign, however there were marriages by soldiers in frontier regions before this edict).

Roman soldiers were forbidden by law to contract a marriage during their period of military service, at least until the time of Septimius Severus

The Marriage of Soldiers under the Empire by Brian Campbell (Published: The Journal of Roman Studies Vol. 68 (1978), pp. 153-166)

In the past there has been considerable difference of opinion off whether or not Roman soldiers in the imperial period were permitted to marry under Roman law. Recent writers, however, seem agreed that all rank-and-file soldiers in the legions, auxiliary brigades, and elite cohorts (stationed mainly in Rome) were denied an official Roman marriage during their term of service, and that the situation changed about A.D. 197 when this right was granted to legionaries at least, by Septimius Severus.

Septimius Severus and the Marriage of Soldiers by Peter Garnsey (Published: California Studies in Classical Antiquity Vol. 3 (1970), pp. 45-53)

The author of this article argues against this view however I am using his statement to prove the generally accepted view of marriage rights being granted under the reign of Septimius Severus.

It has been argued that the legal allowance of marriages amoung soldiers would promote a more stagnant and less mobile military force however there is significant evidence of informal marriages in the provinces between roman legionaries and local women, making this theory questionable however still worth mentioning in conjunction with Gibbons view of the roman military post barbarization.

Overall Edward Gibbon’s arguments are mostly dismissed, the most obvious reason being that the Christian Eastern Roman Empire continued for nearly 1000 more years following the decline of the Western Roman Empire, devaluing his assertion that Christianity degraded the Roman martial values.

Edward Gibbon’s arguments were certainly respected in his day however his view was eventually challenged by J.B. Bury, an author whose writing mostly focused on the Byzantine Empire. Bury’s theses were counter to Edward Gibbons and quite persuasive as he argued the main cause of the decline of Rome was not caused by one singular event or ideology, however that the overall climate in which Rome existed during the 3rd and 4th century were not conducive to its maintained growth. The number of crises faced by Rome during the third century left an indelible mark upon the Empire’s economics and military: mainly the massive decline in trade due to continuous civil war and the truly devastating effects of the plagues which ravaged the empire, namely the Antonine and Cyprian Plagues which severally depopulated the Italic Peninsula.

The explanations of the calamities of the Empire which have been hazarded by modern writers are of a different order from those which occurred to witnesses of the events, but they are not much more satisfying. The illustrious historian whose name will always be associated with the "Decline" of the Roman Empire invoked the "principle of decay," a principle which has itself to be explained. Depopulation, the Christian religion, the fiscal system have all been assigned as causes of the Empire's decline in strength.104 If these or any of them were responsible p309 for its dismemberment by the barbarians in the West, it may be asked how it was that in the East, where the same causes operated, the Empire survived much longer intact and united.