I like to think of the fall of empires on two levels. On the level of specifics, I would suppose there are in each case generally a number of different causes,

I like to think of the fall of empires on two levels. On the level of specifics, I would suppose there are in each case generally a number of different causes, interacting in various ways. On a more general level, empires as such are an unnaturally large and complex form of social organization, and for that reason they inevitably collapse at some point or another.