Rome's inability to tackle a series of barbarian incursions led to the collapse of the Western Empire. Rome was sacked multiple times in the 5th century AD and the last Roman Emperor was formally deposed in 476.
By the early 5th century many independent groups were at Rome’s gates simultaneously. The mishandling of the early Gothic invasions led to a multi-faceted military disaster. The approach of multiple invading armies was a catastrophic event that would have been difficult for any empire to handle.
Many barbarian warbands crossed the Rhine-Danube border into the Roman Empire, causing widespread chaos. Attempts to defend against or integrate these groups failed. The 5th-century invasions were a catastrophic event.
Overstretching led to a reliance on foreign troops
By the 4th century, Rome was struggling to pay for and recruit its enormous military. As a result, it came to rely increasingly on outside militia groups. The Romans attempted to use the Goths who invaded their lands in their army, with disastrous results.
By the 4th century, the Roman Empire's economy was severely damaged by civil strife and runaway inflation. It was never able to solve these economic problems and remained weak in the face of widespread invasions. The decline of Roman material culture began long before the fall of Rome itself.
Rampant Inflation destroyed the Roman Economy
From the 3rd century onwards many Roman emperors would debase the coinage, rendering it nearly worthless. A barter economy returned in many places and people struggled to cope with the now heavy tax burden. The Roman state was broke by the time of the barbarian incursions of the 4th-5th centuries.
Some historians believe the decline of Rome stretches all the way back to the Severan dynasty in the 2nd century when the military emperors eroded the seriousness of Roman high office. Rome became an ineffectual and corrupt empire from then onwards.
Constant Civil Wars
The Roman Empire during the 2nd and 3rd century was increasingly run by military men who undermined the integrity of the state. Multiple military candidates led to constant civil war and economic problems. This model sees a slow decline in stability starting with the Severan dynasty.
A change in fundamental in values may have destroyed the Roman Empire. It has been argued both that Rome was too decadent and immoral, and too pious and Christian by different thinkers. Both groups attribute Rome’s fall to a decline in Roman virtues during the Imperial period.
Christianity broke the spirit of the empire
Christianity has sometimes been blamed for weakening the previously vigorous martial spirit of the Roman Empire. It was most famously proposed by Edward Gibbon, in his Decline and Fall of the Roman Empire.