There were many reasons for the fall of the Roman Empire. Each one intertwined with the next. Many even blame the introduction of Christianity for the decline. Christianity made many Roman citizens into pacifists, making it more difficult to defend against the barbarian attackers. Also money used to build churches could have been used to maintain the empire. Although some argue that Christianity may have provided some morals and values for a declining civilization and therefore may have actually prolonged the imperial era. 
Those morals and values that kept together the Roman legions and thus the empire could not be maintained towards the end of the empire. Crimes of violence made the streets of the larger cities unsafe. Even during PaxRomana there were 32,000 prostitutes in Rome. Emperors like Nero and Caligula became infamous for wasting money on lavish parties where guests ate and drank until they became ill. The most popular amusement was watching the gladiatorial combats in the Colosseum. These were attended by the poor, the rich, and frequently the emperor himself. As gladiators fought, vicious cries and curses were heard from the audience. One contest after another was staged in the course of a single day. Should the ground become too soaked with blood, it was covered over with a fresh layer of sand and the performance went on.
There were many public health and environmental problems. Many of the wealthy had water brought to their homes through lead pipes. Previously the aqueducts had even purified the water but at the end lead pipes were thought to be preferable. The wealthy death rate was very high. The continuous interaction of people at the Colosseum, the blood and death probable spread disease. Those who lived on the streets in continuous contact allowed for an uninterrupted strain of disease much like the homeless in the poorer run shelters of today. Alcohol use increased as well adding to the incompetency of the general public.
One of the most difficult problems was choosing a new emperor. Unlike Greece where transition may not have been smooth but was at least consistent, the Romans never created an effective system to determine how new emperors would be selected. The choice was always open to debate between the old emperor, the Senate, the Praetorian Guard (the emperor's's private army), and the army. Gradually, the Praetorian Guard gained complete authority to choose the new emperor, who rewarded the guard who then became more influential, perpetuating the cycle. Then in 186 A. D. the army strangled the new emperor, the practice began of selling the throne to the highest bidder. During the next 100 years, Rome had 37 different emperors - 25 of whom were removed from office by assassination. This contributed to the overall weaknesses of the empire.
During the latter years of the empire farming was done on large estates called latifundia that were owned by wealthy men who used slave labor. A farmer who had to pay workmen could not produce goods as cheaply. Many farmers could not compete with these low prices and lost or sold their farms. This not only undermined the citizen farmer who passed his values to his family, but also filled the cities with unemployed people. At one time, the emperor was importing grain to feed more than 100,000 people in Rome alone. These people were not only a burden but also had little to do but cause trouble and contribute to an ever increasing crime rate.
The roman economy suffered from inflation (an increase in prices) beginning after the reign of Marcus Aurelius. Once the Romans stopped conquering new lands, the flow of gold into the Roman economy decreased. Yet much gold was being spent by the romans to pay for luxury items. This meant that there was less gold to use in coins. As the amount of gold used in coins decreased, the coins became less valuable. To make up for this loss in value, merchants raised the prices on the goods they sold. Many people stopped using coins and began to barter to get what they needed. Eventually, salaries had to be paid in food and clothing, and taxes were collected in fruits and vegetables.
Wealthy Romans lived in a domus, or house, with marble walls, floors with intricate colored tiles, and windows made of small panes of glass. Most Romans, however, were not rich, They lived in small smelly rooms in apartment houses with six or more stories called islands. Each island covered an entire block. At one time there were 44,000 apartment houses within the city walls of Rome. First-floor apartments were not occupied by the poor since these living quarters rented for about {fckeditor} a year. The more shaky wooden stairs a family had to climb, the cheaper the rent became. The upper apartments that the poor rented for a year were hot, dirty, crowed, and dangerous. Anyone who could not pay the rent was forced to move out and live on the crime-infested streets. Because of this cities began to decay.
During the last 400 years of the empire, the scientific achievements of the Romans were limited almost entirely to engineering and the organization of public services. They built marvelous roads, bridges, and aqueducts. They established the first system of medicine for the benefit of the poor. But since the Romans relied so much on human and animal labor, they failed to invent many new machines or find new technology to produce goods more efficiently. They could not provide enough goods for their growing population. They were no longer conquering other civilizations and adapting their technology, they were actually losing territory they could not longer maintain with their legions.
Maintaining an army to defend the border of the Empire from barbarian attacks was a constant drain on the government. Military spending left few resources for other vital activities, such as providing public housing and maintaining quality roads and aqueducts. Frustrated Romans lost their desire to defend the Empire. The empire had to begin hiring soldiers recruited from the unemployed city mobs or worse from foreign counties. Such an army was not only unreliable, but very expensive. The emperors were forced to raise taxes frequently which in turn led again to increased inflation.
For years, the well-disciplined Roman army held the barbarians of Germany back. Then in the third century A. D. the Roman soldiers were pulled back from the Rhine-Danube frontier to fight civil war in Italy. This left the Roman border open to attack. Gradually Germanic hunters and herders from the north began to overtake Roman lands in Greece and Gaul (later France). Then in 476 A. D. the Germanic general Odacer or Odovacar overthrew the last of the Roman Emperors, Augustulus Romulus. From then on the western part of the Empire was ruled by Germanic chieftain. Roads and bridges were left in disrepair and fields left untilled. Pirates and bandits made travel unsafe. Cities could not be maintained without goods from the farms, trade and business began to disappear. And Rome was no more in the West.
