When looking for the reason for why the roman empire fell it is impossible to just focus on one set of reasons for the collapse of this great empire. We can not focus alone on either internal or external reasons. Instead we must look at both.
The Roman Empire at its peak governed over most of the Eastern world. After the death of Julius Caesar, who had destroyed the Roman Republic, an empire was the easiest was to keep the state going#.
There were many external reasons for this collapse. Most of which seemed to come from the ever increasing size of the empire itself. The Roman Empire of the fourth century A.D. extended entirely around the Mediterranean Sea, including modern Turkey, Israel, Egypt, and North Africa. Modern France (called Gaul) and modern Spain and Portugal (Iberia) were entirely Roman. Modern England was Roman, but modern Scotland and Ireland were barbarian (non-Roman, or non-civilised).
The northern borders of the empire were the Rhine and Danube Rivers. The lands north of these rivers were occupied by a variety of tribes of Scandinavian origin that the Romans called the Germans.
Rome was engaged in border skirmishes with the tribes north of the great European rivers. Strong emperors occasionally extended the empire over the rivers while weak emperors tended to lose those lands. Although the largest organised rival of the Romans was actually the Persian Empire to the east, which occupied modern Syria, Iran, Iraq, and Afghanistan. The Persians were the political descendants of the Parthians who had revolted away from Greek rule following Alexander's conquests and, thereafter, successfully resisted Roman invasions.
The fact that the empire had stretched so much made it more difficult to defend. Maintaining an army to defend the border of the Empire from barbarian attacks was a constant...