Ah, Chinese history, something close to my heart!

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 Three Kingdoms era, together with the preceding long disintegration of Eastern Han is kind of the chivalric age of China (much like the Sengoku period in Japan), romanticised in legends. The most famed battle of this period, the Battle of Red Cliffs in 208, is about to be put to film by John Woo as the most expensive Chinese film ever (according to IMDB, for release in 2007 only). Tho', I'm more fascinated by the immediately pre-imperial Warring States period.

_{*Lunatic*, n.
One whose delusions are out of fashion.}