Historians identify a number of factors that led to the decline and fall of the Han Dynasty. The dynasty is typically said to have fallen in 220 CE.
The first factor that is said to have led to the fall of the Han is one that is similar to a cause of the fall of Rome. This is the fact that the empire became too big. Historians say that the Han had to patrol and defend too much territory. This caused economic problems for them because they could not really afford to pay the soldiers who were needed for this task.
A second factor is related to this one. To defend their empire, the Han increasingly had to pay mercenaries. The mercenaries were not particularly loyal to the Han and therefore did not protect their empire as well as they might have liked.
The third factor that we will examine here was the rise in the power of the aristocracy. The aristocrats were able to take more power for themselves as the Han became weaker. Many peasants then put themselves under the protection of the aristocrats so that the imperial government (which needed a lot of money to provide for defense) could not tax them.
Thus, the Han Dynasty broke down largely because it became too big. It could not sustain the sort of defense needed to protect itself without alienating its own people by imposing excessive taxes and other demands on them. This led to rebellions that eventually overthrew the dynasty.