There were many reasons for the fall of the Roman Empire. Each one intertwined with the next. The same things that restored the Roman Empire caused it to failed such as political authority, of christianity, and the military. 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.
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.
1. 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...