This was mentioned in another thread, but I thought I should give it is own thread just to make sure it's on the radar.
[Disclaimer: I'm still in the early phases of groping around with learning what this game does and doesn't do, so if this is already included I'll be very happy to acknowledge that I've got egg on my face and go off to make an omelette].
The war of the Taiping rebellion was huge, but most of it happened on the fringes of the West's awareness.
Here's a wiki:
http://en.wikipedia.org/wiki/Taiping_Rebellion
British, French, and (believe it or not) American commercial interests were involved in it from the start, but often indirectly in the form of a proxy war.
The foreign merchant community of Shanghai, despairing of the central government's ability to protect them from the Taiping rebels, went out and raised their own private army (the Ever Victorious Army). Since a large part of the merchant community was American (AIG was first founded in Shanghai), they naturally turned to a succession of American commanders for the EVA. For a variety of reasons the EVA didn't have much success until they recruited a British officer on sick leave to be its third commander. The resulting campaigns made Chinese Gordon's reputation, though he is now mostly remembered as Charlton Heston in the movie Khartoum.
In this phase the Western powers were sort of on the side of the Chinese central government, sometimes. A few years later, in the last phase of the Opium Wars, they attacked Peking directly and burned the Summer Palace, an act which has been (rightly) painted as one of the great acts of barbarism of the nineteenth century.
It would be really great to see the triangular yellow Qing flags waving in the game. There's no reason why all the flags you see have to be rectangles.