Don't forget the warlordism of the erly 20th century, when China was essentially many countries for a long period fof time.  

The West needs to get out ahead of this though.  Sometime soon the shit's going to hit the fan, and unless we (the democratic west) counter the nationalist fires stoked by the communists we are going to have  rogue nation the likes of which haven't been seen since  Germany tried to take her place in the sun early last  century.  

The west needs to put pressure on the the Chinese leadership to allow independent unions and the free expression of faith.  I don't expect China to be a democracy anytime soon, but I do think wee nedd to work to establish the insitutions of civil society for a peacecul transition.   We can't change China, but we can make the recogntion of independent unions, and the right to practice your faith freely the entry fee to get Chinese goods into the US and the EU.  

And I'll give my consent to any government that does not deny a man a living wage-Billy Bragg

China has been unified since the 3rd century BCE...

Nope. China has been re-united every time it fell apart after 3rd century BC. (Each time with great cost in human lives, I note.)

Contrary to what others write here, falling apart was not the result of foreign invasion - on the contrary, foreign invaders were sometimes the re-unifiers, most notably the Mongols.

IMO the most notable disintegrations are:

• the Three kingdoms era: East Han empire breaks up into Wei, Shi, Wu (220-280 AD)

• the North-South dynasties era, with more stable Southern and less stable Northern states and dynasties, until reunification by the Shui Dynasty (280-581 AD)

• the Five Dynasties/Ten Kingdoms era, a long-lasting warlord era (this time with the North more stable) between the Tang and Song dynasties (902-979)

• the Southern Sung Dynasty era, when the South was a stable empire but the North progressively fell apart, then all was conquered by the Mongols - Marco Polo visited when the North was already conquered and Kubilai Khan took on the Southern Song (1127-1279)

• The current fascists in place manage to hold the lid on the pressure cooker and maintain themselves in power, same old, same old.
• Unrest grows and China spirals in a repressive vicious circle. The central government becomes ever more authoritarian and possibly ever more belligerent outside its borders as an escape valve for popular anger.
• Unrest grows and, in conjunction with a middle class that's no longer content to just work and consume, forces at least some measure of democratization (the one outcome all would prefer, I presume).

The regional governments have quite a level of competition and the coastal regions have a large prosperity gap with the western areas.

The centre has the army, but the regions are working on that. This will be the key issue in the medium term, naturally enough.