The fall of Rome occurred over many centuries and was caused by several factors including military decay, barbarian invasions, and the failure of the government to respond to these problems.
While these problems existed to a greater of lesser degree, since the end of the 2nd century, their effects were accelerated by the reforms of the emperors Constantine and Diocletian. These reforms changed Roman life as well as the face of the Roman army, moving it away from its classical infantry-based structure to a more cavalry-based system. The army was reorganized into lightly armed troops called "limitanei" who defended the border, and large mobile armies composed of troops called "comitatenses". The border troops were given land to live on around forts they protected. This structure led to farming becoming the job of the border troops so that they could feed as well as protect those on the frontier. Over time, this in turn led to out of date weaponry and neglect in training. The weakness of these troops meant that more mobile troops were needed to compensate, and an easily penetrable border as a result of the weakness led to the need for highly efficient mobile armies. Since the cavalry were the most mobile unit of the army, they began to be the favored military unit. With forces strung along the border and concentrated large mobile armies, an increased number of recruits were required; however, land owners were reluctant to let themselves or their kin be recruited because that left less workers for their farms. At the same time, the division of the empire into outer imperial provinces and inner provinces controlled by the Senate had its own effect. Since the armies largely remained in the outer imperial provinces, the people of the inner provinces were out of touch with the army and were no longer attracted to service, again reducing the available pool of recruits. One reason that many avoided Army service was because Roman citizenship was now offered freely, where in prior times military service had been a path to citizenship. The result was less manpower available for Rome. The Roman army was left with no choice but to recruit barbarians, who could in this way both find employment where they had no skills, and hope to obtain Roman citizenship. At the same time the weakness of the border troops had allowed more barbarians to migrate into the empire then ever before. Not all of these were friends of Rome, or potential recruits.
The admittance of these barbarians into the army throughout the fourth century helped to lay the beginning foundations for the complete "barbarization" or "germanization" of the Roman army. There were two types of barbarization. The first consisted of recruiting individual barbarians into Roman units. There was no systematic opposition to recruiting or promoting these men as individuals into the legions; they did make it to higher ranks, and had been a part of the empire since the first century BC. The second type of barbarization involved the short-term use of entire barbarian groups or allies. They served as integral units supplementing Roman forces, many times even during Roman civil wars. Barbarians who became a part of Rome in this way were given "federate status," which meant that they were under their own rulers, paid no taxes, and were allowed to carry arms among mostly disarmed Romans. Federate troops fought under their own commanders and were not subject to Roman training and discipline. "Many attribute the collapse of infantry morale and discipline to the influence of these federate barbarians" (Ferril, 129). Besides clothes, shelter and the other necessities of life, some barbarian groups were also given weapons factories by Rome. However, this left the Romans susceptible to attack by the now well-armed barbarian groups within the Empire, who were becoming stronger as a result of Rome's own actions.
By the 4th century AD, the Roman military had become noticeably weaker, especially to those barbarians who had for years been penetrating Rome's borders. Soon barbarians began to infiltrate the Western Empire on an even larger scale then they had previously done, first as small groups of settlers and mercenaries, but eventually as entire tribes. They detected military problems and began to establish small kingdoms (e.g., Gaul, Spain). This was the beginning of the Germanic kingdoms; many of whom were within Rome's own borders. The empire was becoming less and less organized and, as time passed, the Germans began to assert themselves more and more, the Empire was beginning to fall apart.
Changes in the barbarian world also had their effect on the Empire. The development of eastern Germans had for centuries been stimulated by undisturbed contact with the frontiers of the ancient "civilized" world. Contact with the Mediterranean through the amber trade encouraged development from a peasant culture. Unfortunately, the economy was still unable to support their needs, and overpopulation and pressures from the East resulted in more border incursions. During the Iron Age, the Germanic peoples were cut off from the Mediterranean by Celts and Illyrians. This, and population increases, drove them at first farther south, against the Empire's borders and then into the Empire itself. Rising floodwaters drove Cimbri, Teutoni and Ambrones from Jutland, modern Denmark, through Celtic/Ilyrian zones to reach the edge of territory with Roman influence. Visigoths moved into Spain, Franks into France, Burgundians into lower France, and Goths into Italy. They disrupted trade almost entirely. They destroyed village life. Rome's weak border forces could not prevent this migration, and the mobile armies were usually too far away to react in time, and could not react to all threats at once. As a result of the multiple challenges across a broad front, Rome's army needed to grow. Unfortunately for Rome, it had become very hard to get Roman recruits. Due to this, Emperors were now forced to let even more barbarians into the army to help defeat the other bands of barbarians terrorizing the frontiers.
A series of civil wars helped destroy Rome from the inside-out during the later centuries of the Roman Empire. The centralized mobile armies over time had become loyal to their general first, and then to Rome. Warring generals were now using their armies against one another in contests of power, often for the imperial throne of Rome itself. Sadly for Rome, this meant that they were unavailable to control the massive civil unrest or respond to multiple incursions on the Roman frontier. The civil unrest during the latter centuries of the Empire was caused by several things, among them the political turmoil that was going on in Rome and the breakdown in trade due to barbarian incursions (itself brought on in part by the weakening of the army at the border). Political intrigue and the warring generals led to the frequent rise and fall of emperors, and unstable leadership at the center. The numerous civil wars also disrupted agriculture and trade, especially in the western part of the empire, leaving more people without homes, and less food to feed the hungry, leading to new mutiny by the people of Rome.
In the last half of the 4th century, Romans suffered two major defeats at the hands of the Germans and Persians which weakened their military forces significantly; after these wars the Roman military was soon almost incapable of protecting its empire at all. In the east, Rome's borders had always been uneasy. The Parthian, or Persian, Empire lay beyond, and Rome had never been able to subjugate it. In the late third century, this Persian Empire became actively hostile, and marched against the Romans. In a series of battles the Persians forced the Romans out of Mesopotamia. At roughly the same time, other migrating tribes (Tartars and Mongols) caused the Huns to move west, out of the steppes and into the lands of the Goths. The Huns pushed Ostrogoths west against Visigoths. Diocletian, Constantine, and Valentinian I kept the barbarians beyond the borders, but the push from the Huns ultimately drove the Goths into the Empire. The small barbarian tribes who had opposed the early empire now banded together to form more powerful confederations. The Goths united under single leadership, and defeated the army of Valens at the key battle of Adrianople in 378. From this point on the Romans could not destroy the Goths. Rome's forces were seriously weakened by the defeat at Adrianople, and it was now easier to penetrate Roman borders. "Barbarian pressures on the frontier were a decisive force in the fall of the Western Roman Empire" (Ferril, 15). Interestingly, the defeat at Adrianople was partly a result of a skilled Goth cavalry that was actually Roman trained. These two defeats did not end the empire, but did severely weaken it.
Theodosius was the last emperor to rule a unified Roman Empire. When he died he left two sons, Arcadius and Honorius. Arcadius became Augustus in 383 and was installed at Constantinople. Soon Honorius was made Augustus too, with responsibilities for the Western Empire. Both emperors were very young and under the constant influence of advisors. For Arcadius it was the Praetorian Prefect, Rufinus, and for Honorius it was Stilicho. Unfortunately Rufinus and Stilicho were hostile towards each other and the little unity that was left within the empire was in danger. The eastern and western Empires were soon warring.. The western army was now only a shadow of its former self; Arcadius had no defenses against the continuing invasions of his territory by the Goths. His problems got so bad that he had to notify Roman Britain that Roman troops stationed there would be withdrawn to save Rome itself. Arcadius had to face a strong leader of the Visigoths named Alaric. Alaric and his tribe initially helped to defend Rome's borders, but soon Alaric recognized Rome's weakness and turned against her. He invaded Italy in 401, but was turned back. He was briefly allied with the West again in 407, but led a second invasion of Italy in 408 that was to culminate in the sack of Rome in 410. Barbarians soon began forming their own nations on Roman soil.
The actual end of the Western Roman Empire is marked by the usurping of the Roman throne by Odoacer; a Germanic chieftain. He and his soldiers were formerly mercenaries for Rome when, in 476 a Germanic tribe, the Hiruli, revolted and named him king. Soon afterward he took control of Rome and deposed the last Emperor of the Western Roman Empire.
The fall of Rome happened over a long period of time spanning over three hundred years. Several related issues contributed their part to the empire's downfall. The army's deterioration, its continuous involvement in civil wars, the rise and fall of numerous emperors, the destruction of farms and villages throughout the fourth and fifth centuries and the pressure from invading peoples from outside the empire as well as within, all contributed. These were the major factors leading to the empire's downfall.