What was the cause for the collapse of the Roman Empire? How were these similar and different to the collapse of the Assyrian, Hellenistic and Han dynasties and why were they similar and different? Actually this is a diffucult question because there are so many parts that go along with it.
In the same way, the Han dynasties were trying to include vast areas within their empire. Over time, the distances simply became overwhelming, as cultures from different regions could not remain unified behind one single ruler. In modern times, there are some who argue that this problem is now obsolete due to the instantaneous communications available today. I'm not convinced this is the case, however, or that such rapid connections are always for the best.
I would argue that one of the central reasons that led to the Roman Empire's collapse was the problems that it had in controlling such a vast territory. In many ways, its success was part of its downfall, as the logistics of managing an empire that stretched all the way from Britain to North Africa were massive and meant that the empire was vulnerable to attack along the vast expanses of its boarders.