The Roman Empire (Latin: Imperium Romanum) was the post-Republican period of the ancient Roman civilization, characterized by an autocratic form of government and large territorial holdings around the Mediterranean Sea in Europe, Africa, and Asia. The 500-year-old Roman Republic, which preceded it, had been destabilized through a series of civil wars.1 The main popular reasons for the decline of Roman Empire are discussed in the following paragraphs.
The most straightforward reason for the same has been the invasion by Barbarian tribes. Rome had tangled with Germanic tribes for centuries, but by the 300s “barbarian” groups like the Goths had encroached beyond the Empire’s borders. The Romans weathered a Germanic uprising in the late fourth century, but in 410 the Visigoth King Alaric successfully sacked the city of Rome. The Empire spent the next several decades under constant threat before “the Eternal City” was raided again in 455, this time by the Vandals. Finally, in 476, the Germanic leader Odoacer staged a revolt and deposed the Emperor Romulus Augustulus. From then on, no Roman emperor would ever again rule from a post in Italy, leading many to cite 476 as the year the Western Empire suffered its deathblow.
There also occurred a severe financial crisis, which resulted in Economic troubles and overreliance on slave labor. Constant wars and overspending had significantly lightened imperial coffers, and oppressive taxation and inflation had widened the gap between rich and poor. But when expansion ground to a halt in the second century, Rome’s supply of slaves and other war treasures began to dry up. A further blow came in the fifth century, when the Vandals claimed North Africa and began disrupting the empire’s trade by prowling the Mediterranean as pirates. With its economy faltering and its commercial and agricultural production in decline, the Empire began to lose its grip on Europe.
The fate of Western Rome was partially sealed in the late third century, when the Emperor Diocletian divided the Empire into two halves—the Western Empire seated in the city of Milan, and the Eastern Empire in Byzantium, later known as Constantinople. The division made the empire more easily governable in the short term, but over time the two halves drifted apart. East and West failed to adequately work together to combat outside threats, and the two often squabbled over resources and military aid. The strength of the Eastern Empire served to divert Barbarian invasions to the West. Emperors like Constantine ensured that the city of Constantinople was fortified and well guarded, but Italy and the city of Rome—which only had symbolic value for many in the East—were left vulnerable.
Another reason for the same was the overexpansion and military overspending and empire faced an administrative as well as logistical nightmare. They lacked proper communication as they struggled to marshal enough troops and resources to defend its frontiers from local rebellions and outside attacks, and by the second century the Emperor Hadrian was forced to build his famous wall in Britain just to keep the enemy at bay. As more and more funds were funneled into the military upkeep of the empire, technological advancement slowed and Rome’s civil infrastructure fell into disrepair.
There had been no other factor than Government corruption and political instability to have worsened the situation. The Praetorian Guard—the emperor’s personal bodyguards—assassinated and installed new sovereigns at will, and once even auctioned the spot off to the highest bidder. The political rot also extended to the Roman Senate, which failed to temper the excesses of the emperors due to its own widespread corruption and incompetence. As the situation worsened, civic pride waned and many Roman citizens lost trust in their leadership.
The arrival of the Huns and the migration of the Barbarian tribes is also a reason for the decline of Rome. With their migration, they drove many Germanic tribes to the borders of the Roman Empire. The Romans grudgingly allowed members of the Visigoth tribe to cross south of the Danube and into the safety of Roman territory, but they treated them with extreme cruelty.
The decline of Rome dovetailed with the spread of Christianity, and some have argued that the rise of a new faith helped contribute to the empire’s fall. While the spread of Christianity may have played a small role in curbing Roman civic virtue, most scholars now argue that its influence paled in comparison to military, economic and administrative factors.
During the decline, the makeup of the once mighty legions began to change. Unable to recruit enough soldiers from the Roman citizenry, emperors like Diocletian and Constantine began hiring foreign mercenaries to prop up their armies. The ranks of the legions eventually swelled with Germanic Goths and other barbarians, so much so that Romans began using the Latin word “barbarus” in place of “soldier.” While these Germanic soldiers of fortune proved to be fierce warriors, they also had little or no loyalty to the empire, and their power-hungry officers often turned against their Roman employers.
The effects of the fall of Rome were huge. Rome was a huge empire, which controlled almost all of Europe. When it fell, the territories it had controlled had no government, and the people had to learn to fend for themselves. This was a huge and tragic change for the people involved. They had depended on the Roman government to defend them, and under Roman law, many of them weren't even allowed to learn how to fight. When the Roman army abandoned them, they were left to defend themselves against barbaric invaders who attacked them from all sides. MANY (perhaps the majority) of people had their villages pillaged, and their women raped.
n top of that, there was a second, devastating result. The people living in those territories were largely peasants whose job was to work the land. It wasn't like today, where people living in pretty much any part of the country can be doctors, engineers, and so on. Back then, the local peasants worked the land, and any doctors or engineers, pretty much anyone who knew how to do anything besides work land, came from Rome. When these people evacuated, the peasants had no knowledge of how to build or repair walls or roads on their own, how to make their own weapons, how to fight, etc. etc. It's basically like if someone put you and your family in the middle of Afghanistan today and told you "Okay you're on your own, fend for yourself."
All of these factors combined to form the Dark Ages, a period which lasted for roughly a thousand years. Different areas of land came under small, local control by whichever local land owners could afford to hire men to help fight off the invaders. Often time these were cruel warlords who abused their power. Even so, the people didn't know how to fix anything on their own, and were barely literate. Well into the medieval ages, the only cities in existence were inside older, Roman towns, many with walls that gradually crumbled over time, and which no one knew how to fix.
In addition, people also didn't know how to fix roads, and so different cities were cut off from each other as roads broke down. Traveling became a very dangerous activity, particularly at night, and gradually people fell into a long period of superstition and ignorance, whereby they began imagining that "dragons" and other monsters lay outside the towns and would eat anyone who traveled at night. The fact traveling outside at night was dangerous (and therefore people who traveled at night tended to disappear) reinforced this superstition. Gradually, tales arose of brave knights who went out into the forests and battled these monsters and discovered all sorts of magical talismans and so on.2
But though the Empire itself no longer existed, through the Christian Church, through the always idealized vision of glorious Rome, and through the political structures that evolved out of Rome's carcass, vestiges of the Empire played vital and identifiable roles in the formation of the early Medieval European world. In other words, the result was that the Medieval Ages began.
Illustrated History of the Roman Empire:
Society after fall of Roman Empire: http://www.mariamilani.com/ancient_rome/society_after_roman_empire_fall.html
A History of Western Society by McKay, Hill, and Buckler, vol. I, 9th edition