1 – Internal Strife
In ‘The History of Decline and Fall of the Roman Empire’, Edward Gibbon had a controversial theory. He claimed the rise of Christianity contributed to the fall of Rome as it bred a ‘turn the other cheek’ mentality. He also claimed the religion valued idle and unproductive people and also led to internal divisions. Gibbon wrote this in the 18th century, and modern historians tend to disagree with his analysis.
Gibbon’s claim that Rome was subject to moral decay probably holds more water. In the 2nd Century BC, Polybius wrote of a decline in moral virtue that led to the fall of the Republic. The same affliction appeared to damage the empire. The original ideals, values, and traditions upon which Rome was founded declined and were replaced by a notion that life was cheap and depravity, gluttony, and cruelty were the norm.
The Senate acted as an advisory body, but corrupt and power-hungry rulers routinely ignored this advice. Angry senators would plot against the leader and decisions were never made for the good of the empire. The Praetorian Guard were the personal bodyguards of the emperor, but they too became drunk on power. Eventually, they decided who would be emperor and would routinely murder the person on the throne.
The Third Century Crisis almost destroyed the Empire and paved the way for its eventual downfall. From 235-284 AD, there were at least 26 emperors and all, but a handful was murdered. Rome’s traditional trade network collapsed during this period so by the time Diocletian brought an end to the Crisis; the Empire was on its last legs.
2 Economic Deterioration
As is the case with all great empires, Rome was founded on a strong economic foundation. By its peak in 117 AD, the empire spanned approximately 1.5 million square kilometers and was inhabited by around 130 million people. Trade was crucial to the growth of the Roman economy and its outstanding transport system allowed a huge variety of goods to be imported across its borders. However, governing the empire was an expensive task with enormous military, administrative and logistical costs. A combination of several factors resulted in hyperinflation, heavy taxes, a localization of trade and ultimately, a crippling financial crisis.
The denarius was the main coin used in the first 200+ years of the empire. When it was created, each coin contained 4.5 grams of pure silver and was the equivalent of a day’s wages for the average craftsman/skilled laborer.
Nero was one of the first emperors to devalue the denarius, and by the time Gallienus took the throne in 253 AD, the coins contained approximately 5% silver and consisted of a bronze core with a thin layer of silver. By 265 AD, the denarius contained 0.5% silver; the result was inflation of up to 1,000% across the empire. By this time, Rome had no more enemies to steal from so taxation was raised. The resulting mess completely paralyzed trade. By the end of the 3rd Century AD, the vast majority of trade was localized with barter methods used instead of the exchange of currency. To make matters worse, the empire faced an intense labor deficit. For centuries, the empire relied on slaves to work as craftsmen and till the fields. In the latter part of the Republic and the first 150 or so years of the Empire, constant conquest meant there was a steady supply of slaves. By the 2nd Century AD, Roman expansion had practically ground to a halt, so the supply of slaves dried up along with the accompanying plunder. Later on, there was a growing humanitarian sentiment within the empire (when Christianity became the main religion) as Romans began to realize that slavery was wrong. A significant proportion of slaves were freed; this exacerbated the labor problem.
3 – The Split Empire
When Diocletian became emperor in 284 AD, he realized that it was no longer possible for one man to rule the vast empire. He made Maximian his co-emperor sometime in 285 AD with Empire split into east and west; Nicomedia was the original capital although it was changed to Constantinople in 330 AD. From this point onwards, the Senate would be ignored. In the past, it had advised the emperor on matters of state. The power of the empire was supposed to be based on an active military which became a problem when the empire’s military strength waned.
Some of the later emperors never even set foot in Rome and while the division offered an initial boost in strength, it ultimately made the Western Empire more vulnerable to attack. Rome had been having issues with Germanic tribes for centuries but was usually able to repel them or buy them off with land, an invitation to join the army or citizenship. The split became permanent in 330 AD.
Both halves of the empire prospered equally until the defeat of Eastern Emperor Valens at Adrianople in 378 AD. East and West effectively broke apart due to internal and external problems during the reign of Theodosius  the West.
