By constantly invading other lands to gain power in some aspect, both empires spread themselves thin over a large area. By doing this, they effectively cut lines of communication from the capitol to the outer extremities. Often times the satellite of an empire would be so far away that a corrupt general might declare himself ruler of it, and establish his own laws and regulations. This did, however, benefit other cultures that sprang from the death of such a superpower like the Anglo-Saxons in the case of Rome. When Rome was starting to fall and needed more support, the empire acted as a human body would when having a stroke (most of the blood from the limbs and other parts moves to the heart and brain to support them) and all of the satellites came down to Rome, unsuccessfully. Nevertheless, With the Romans gone but all their architecture there, the Anglo Saxons could move into the former land of the Romans, the former land of the Celts.
To Contrast these two empires is by far the more complex task and it puts a murky cloud around the answer as to why they fell. The story of these empires would have one believe that failure is predestined for all that is great no matter what provisions are taken. The major differences between the two civilizations are in their culture, economy, and ethnicity. The Han Empire did not integrate other cultures into its homogeneous society, whereas the Romans would use slaves from different cultures and allow anyone under the empire to be racially diverse. Rome's heterogeneous society, it is argued, did not necessarily happen on purpose, it began with the barbaric slaves slowly breeding and integrating themselves into the culture. This is not to say China did not have slaves, but for some reasons (mostly nationalistic zeal I suppose) the slaves did not perform an integration as did the Germanic barbarians.
Although with almost identical foreign policy doctrines, the military systems were almost completely different.
