Watchman... good point.

In many ways, the analogy to Rome is similar to the Islamic situation. They went through great expansion but when internal and external strife, economic difficulties, and so on set in, they saw a decline in society. Though parts of it still survived and would not decline (militarily at least) for many more years - such as the Ottoman Empire, which would not be stopped until Vienna in 1683, society had declined noticeably. That analogy could be that whereas Rome had the Byzantine Empire survive and carry on its legacy, and it would not decline military for many more years, it could never get back to its old status of dominance and culture.