However, Northern India was quite often united under a single Empire. The only difference between the string of Empires in Northern India and the Chinese Empire is that unlike the Chinese, we consider a change of dynasty in charge as marking the "fall" of that civilization.
fasquardon
Generally speaking, I think this is a bit of an exaggeration. China's periods of division, aside from the long Song - Jin - Liao tended to be shorter: north India was divided into multiple states at least as often as it was unified, I don't think we get anything like "continuity of succession" until the Sultanate of Delhi. And when we talk about "Northern India", we're basically talking about the Indus-Ganges river complex, a geographically rather more compact area than China. As well as the Dravidian south, Central India/the Deccan was usually mostly outside of these empires.
Now, if you want to say "there's a certain natural unity to the Ganges drainage area", I'd agree with you. Anyhoo, we're drifting from the subject of Iran/Central Asia. I'd say the famous "unconquerability" of Afghanistan is really a product of modern history: before the emergence of the modern Pathan-dominated state, Afghanistan had usually been under the rule of one empire or another. Olde Persians, Maruyans, Kushans, Hephthalites, Ghurids, Mongols, and Timurids. If the Persians/Timurids have Afghanistan, then they're in a good position to master the Punjab, which gives them a lot of added population and resources (perhaps not relatively as much as they have today: IIRC, in the last couple centuries the population of the Pakistan area has grown faster than the Indian subcontinent as a whole). Extending further into India is iffy: Iranian or Turkish military commanders aren't members of the British East India company, and the longer and slower the line of communications from Samarkand or Esfahan or whatever[1], the greater the odds they will break away to carve out their own states. [2]
Bruce
[1] Man, Persia has had a lot of capitals.
http://en.wikipedia.org/wiki/Capitals_of_Persia
[2] I'm assuming no Mughals of course.