What is a period of disunion?
Periods of disunion are considered exceptions to the rule, flaws in cosmic order, by traditional Chinese thinkers because of the cultural assumption that the world should be united under the rule of the Son of Heaven.
The core rationale of traditional Chinese government, ever since the early Zhou (c. 1025 BCE) has been the idea of the Mandate of Heaven. This holds that Heaven grants a single family the right to rule the human world on its behalf, conditional on competent rule. If the people suffer under bad rule, Heaven will withdraw its Mandate and chaos will ensue until a unifying ruler proves by his success that he has gained a new Mandate from Heaven. Thus, a period of disunion is a lapse of the cosmic order, an anomaly that must be put right as soon as possible.
The most notorious of the historical periods of disunion was that of the Warring States. This began around 480 BCE when Zhou rule went into its final decline, and ended when China was unified by the Qin in 221 BCE. Qin was followed quickly by Han, but between the fall of Han in 220 AD and reunification under the Sui in 581 was the Wei-Jin-Northern and Southern Dynasties period, when central rule was at best weak. Sui was quickly replaced by Tang, but when the Tang fell there were the Six Dynasties (907-979). The Song Dynasty was established in 960, but some would see the Song itself as the last great period of division, since North China was soon lost to barbarian rule.
The end of the Han dynasty was marked by the separation of the large families of that dynasty. The families took advantage of the weakened state of the government and started to establish their own private armies. Many dynasties were established during this time. This was a period of disunion.