The Khitan exodus, one of the most improbable awesome story you probably never heard.

[URF8]

Why is Han China able to absorb and assimilate foreign invaders who occupy the country for protracted periods of time? Han China seems to change the invaders rather than being changed itself.

Other invaders change the people they occupy (e.g., the Arab invaders of Syria) or are both transforming and transformed (e.g., the Norman invaders of Anglo-Saxon Britain). But this has not happened to the Han.

I wonder if it has something to do with sheer numbers and the power of Han culture?

[RollingWave]

Well, the truth is that Han Chinese was changed as well, China during the Han / Tang / Song / Ming were all quite different in various ways, and thre are plenty of things you see today as Chinese that actually had central Asian or Indian origins (like many traditional Chinese instruments).

For example, today we think traditional Chinese as being rather chauvinism, but during the Tang period wealthy women very commonly rode around in horses and sometimes had multiple partners, which is a rather clear nomadic legacy as the Tang itself can be traced from nomadic kingdoms that spawned up in the wake of the destruction of the Han era.

But the thing is that you need to realize the Chinese were typically invaded by folks with far less culture than they, many were tribes that didn't even have a writing system, let alone written literatures. and the population difference is extreme. usually 1:100 or more, so it's pretty hard for foreign culture to assilimate China completely.

Usually when certain cultures are really assilimated (like say.. Ancient Egyptian by the Romans) it happened because the Roman empire was both much bigger than Egypt, and it's culture base was not significantly less powerful. and even then it was really Christianity that merged both of them into a similar new culture anyway that was neither Roman nor Egyptian. We seem to assume that the Turks assilimated the Byzantiums but in reality it seem ot have been more of a even thing, as they simply formed a new culture together that's Turk in name but compose of many Byzantium elements.