@TheHoarseWhisperer: good point with the environmental contribution. I'd still be leery of crediting the environment's contribution as being resposible, but I can totally see how it might make it easier for an invader to establish themselves, or for an extant empire to fend off invaders.

Quote Originally Posted by TheHoarseWhisperer View Post
I don't know much about the Omayyad or Rashidun, but is it possible that their internal problems arose from the expense of maintaining a large empire?
Not really, at least not for the Rashidun. That collapse was mostly political intrigue. That first caliphate was ruled by the immediate successors to Muhammad: Abu Bakr, Omar, Uthman, and Ali, and there was a succession debate right off the bat between Abu Bakr and Ali. The Caliphate collapsed when Ali's son Hassan ceded the throne to Omar's powerful cousin Muawiyah (who ruled from Damascus, and had quite the army at his command). When Muawiyah died Hassan tried to re assume the throne, but Muawiyah's son Yazid killed him, cementing the new Omayyad Caliphate in Damascus.

But, I suppose the Omayyad fall could be seen as overreach. So, fine. You win. =P However, the problem was less expense, and more oppression/poor governance in the fringes/cultural differences with the fringes. Which I suppose is a good additional reason for collapse, cultural incohesion.