What political factors helped to cause the collapse of the Han Dynasty?

One of the major political causes of the collapse of the
Han Dynasty was the series of purges that it carried out in attempts to suppress
challenges to the Han emperors.

As the dynasty's power
declined, there arose a number of attempts, often by the relatives of empresses, to take
power from the emperors.  The emperors tended to meet these efforts with large-scale
repressiosn of those who supported the rebellions.  These purges led to a great deal of
dissatisfaction within the elites of the society.

Another
problem was the fact that the largest of the politically powerful families were simply
too big to be controlled by the central government.  As the emperors lost power, these
powerful families took more power for themselves.  They were eventually able to split
the empire into three separate empires.  This was a major step towards the total
disruption of the Han Dynasty.