Why a society goes into decline and eventual oblivion is one of the most complex, interesting, and important questions one can ask in history. The decline and fall of the Roman Empire has especially fascinated historians down through the centuries. How could the most powerful empire in antiquity just come apart at the seams and disintegrate? While historians have focused on various causes ranging from barbarian invasions and moral decadence to the influence of Christianity and lead poisoning, the fact is that many factors combined to lead to the downfall of Rome and open the way into the Middle Ages. Furthermore, these different factors fed back on one another to aggravate the situation and also to make the process of decline more complex to trace.
The first signs of trouble came in the reign of Marcus Aurelius (16l-180 C.E.), the last of the so-called "Good Emperors". When he came to the throne, the Roman Empire still seemed to be experiencing a golden age. The government was efficient, fair, and honest. The army secured the frontiers from invasions. And the economy was healthy in both the countryside and cities. However, during Marcus' reign things started to fall apart. There were five major problems feeding into Rome's decline.
Two problems were of an especially long-range nature dating back to the time of Augustus. One was that few new provinces were added to the empire during the Pax Romana, thus providing Rome with few new sources of revenue. Another drain on the economy was the growing volume of trade with the East for such luxury goods as silks and spices. Silk came all the way from China through a multitude of middlemen and cost its weight in gold, causing a tremendous amount of gold and silver to leave the empire to pay for these luxuries.
A third problem was a devastating epidemic spread throughout the empire by victorious legions returning from a war with the Parthians in the East. Historians then, having little understanding of such phenomena, concentrated mainly on individual people rather than on larger forces, such as disease, affecting history. Therefore, we have little information on what this plague was (possibly smallpox), what its symptoms were, and how many people were affected. If the plague destroyed a significant part of the population, say 10% or more, then it may have been an important factor in the decline of the Roman Empire. Since this was not a mechanized society, most of its labor and energy came from people. If many of those people were lost, society was in trouble. The greater number of labor saving devices such as waterwheels being used from this time on seem to indicate there was a serious population loss. Disease would be a major candidate for its cause.
The fourth major problem Marcus Aurelius faced was barbarian invasions. Apparently population pressures were building among the various nomadic and semi-nomadic tribes beyond Rome's frontiers. At the same time, extended contact with Rome had taught many of these tribes how to combine into larger more effective confederations for fighting Rome. The result was a massive invasion by a tribe known as the Marcomanni across the Danube frontier, with some of these invaders even making it all the way into Italy. The problems of defense were complicated by the fact that the legions were weakened by sickness. The effort to drive the invaders out was so desperate and recruits were so hard to find that even slaves were enlisted. Eventually, the frontier was restored, broken through again several years later, and restored again. By Marcus Aurelius' death in 180 C.E., the empire's population, army, and economy were exhausted by the tremendous efforts of the past two decades. Although the frontiers were restored, pressure from the tribes on the frontiers continued to grow. This required a larger army to defend the frontiers, more taxes to support that army, more bureaucrats to collect those taxes, and even more taxes to support those bureaucrats.
A fifth problem was that, after Marcus Aurelius' death in 180 C.E., men unworthy of the throne generally ruled Rome. For example, there was Marcus Aurelius' son, Commodus, who spent most of his time racing chariots and fighting gladiators in the arena instead of facing the important problems of ruling. Most of these emperors met violent ends, either through court intrigues or military mutinies. One common and unfortunate pattern these emperors followed in order to keep their thrones was to give ever increasing bonuses to the army to keep it happy, thus heaping another huge burden on the Roman economy. Despite all this, the illusion of eternal Rome persisted in people's minds.
So many drains on the economy left the Roman government short of money. Therefore, it raised taxes and started debasing the coinage (i.e., decreasing its gold and silver content). This led to inflation, causing the soldiers to demand more pay to meet their expenses. The government thus faced more money shortages, leading to more taxes and coinage debasements, and so on. To make matters worse, this process triggered an even more serious cycle that left the empire in chaos for fifty years.
At the center of this new cycle were rebellious troops who would overthrow an emperor and put their own generals on the throne in order to get a raise in pay. While some criticism for the troops' actions is justified, we should keep in mind that coinage debasement and the resulting inflation were destroying the buying power of the their salaries. They felt they had to do something to protect their incomes. However, the resulting civil wars stripped the frontiers of troops as they marched to Rome to put their general in power. This in turn invited invasions by the tribes to the north and Persians to the East. The resulting civil wars and invasions would further ruin the economy. This, of course, made it hard to pay the troops who therefore rebelled again, leading to more invasions, more economic problems, and so on. Complicating all this was a new epidemic (possibly measles) that hit the empire around 250 C.E. Meanwhile, all this would feed back into the ongoing cycle of coinage debasement discussed above, which then generated more revolts, civil wars, invasions, etc.
The fifty-year period starting with the reign of Maximinus the Thracian in 235 C.E. was one of the most turbulent and chaotic periods in history, making it extremely difficult to discuss in any detail. At one point, eighteen different men were each claiming they were the emperors of Rome. At the low point of these troubles, the Emperor Gallienus controlled no more than Italy, Greece, Illyria (modern Yugoslavia), and North Africa.
Many of the invaders crashing across the frontiers were new tribes, such as the Goths, whom growing population pressures had forced to migrate toward the Roman Empire. Since these newcomers had little or no prior contact with Rome, they looted and plundered with incredible ferocity, murdering thousands of helpless people whose only crime was being in the path of conquest. Parts of the empire that had seen no wars for centuries were subjected to devastating raids while the army was largely busy making and unmaking emperors. To the East, a new and more aggressive neighbor, Sassanid Persia, had replaced the Parthians. The Persians probably would have overrun the whole eastern half of the Roman Empire, except that the independent oasis city of Palmyra stopped them and then basically ruled the East for itself.
Luckily, a series of remarkably tough and capable emperors emerged from Illyria to restore the Roman world's boundaries. The most important of these emperors, Aurelian (270-275), attacked and destroyed Palmyra and its famous queen, Zenobia. This restored the eastern frontier. Aurelian then reclaimed Gaul, Spain, and Britain to restore the Western frontiers as well and earn himself the title: "Restorer of the World". Despite the remarkable accomplishments of Aurelian and the other Illyrian emperors, they were all murdered by their own troops. Finally, in 284, an even more remarkable emperor, Diocletian, came to the throne and started to put the empire back on its feet. It was this emperor who put an end to the half-century of anarchy that had come close to destroying the Roman Empire.