There are several reasons, and here are some of them
When the Roman Empire started, there was no such religion as Christianity, although by the time of the second emperor, Jesus had been executed for treasonous behavior. It took his followers a few centuries to gain enough clout that they were able to win over imperial support. This came in the early 4th century, with Constantine, who was actively involved in Christian policy-making. Over time, Church leaders became influential and took power away from the emperor; for example, the threat of withholding the sacraments compelled Emperor Theodosius to do the penance Bishop Ambroserequired. Since Roman civic and religious life were the same -- priestesses controlled the fortune of Rome, prophetic books told leaders what they needed to win wars, emperors were deified, Christian religious beliefs and allegiances conflicted with the working of empire.
2. Barbarians and Vandals
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Rome embraced the barbarians, a term covering a variety and changing group of outsiders, using them as suppliers of tax revenue and bodies for the military, even promoting them to positions of power, but Rome also lost territory and revenue to them, especially in northern Africa, which Rome lost to the Vandals at the time St. Augustine.
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One can spot decay in many areas, going back to the crises of the Republic under the Gracchi, Sulla and Marius, but in the imperial period and in the military, it meant men were no longer trained right and the invincible Roman army was no longer, and there was corruption throughout.
Right now, the price of an ounce of gold is $1535.17/ounce (EUR 1035.25). If you bought what you thought was an ounce of gold and took it to an appraiser who told you it was worth only $30, you'd be upset and probably take action against the gold seller, but if your government issued money that was inflated to that degree you would no more have recourse than you'd have the money to buy necessities. That was what inflation was like in the century before Constantine. By the time of Claudius II Gothicus (268-270 A.D.) the amount of silver in a supposedly 100% silver denarius was only .02%.
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The presence of lead in the drinking water leached in from the water pipes, glazes on containers that came in contact with food and beverages, and food preparation techniques could have contributed to heavy metal poisoning. It was also absorbed through the pores, since it was used in cosmetics. Lead, associated with contraception, was recognized as a deadly poison.
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Economic factors are cited as a major cause of the fall of Rome. Some of the major factors, like inflation, are discussed elsewhere. But there were also lesser problems with the economy of Rome that combined together to escalate financial stress. These include:
- Poor management,
- The dole (bread and circuses), and
7. Division of the Empire
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The Roman Empire was split not just geographically, but culturally, with a Latin Empire and a Greek one, the latter of which may have survived because it had most of the population, a better military, more money, and more effective leadership.
8. Hoarding and Deficit
Causes of the fall of Rome include economic decay through hoarding of bullion, barbarian looting of the treasury, and trade deficit.
9. Want Even More?
The University of Texas has reposted a German list ranging from the puzzling to the obvious with a bunch of good ones in between: "210 Reasons for the decline of the Roman Empire." Source: A. Demandt, Der Fall Roms (1984)