In Qin Hui's "Breaking Out of the Imperial System," there is an observation: the main reason for the population decline in the West is plague, while in China, it is closely related to the change of dynasties. Looking at the reasons for population decline in Chinese history, disasters are not even ranked, as human conflicts are much more terrifying. Opening a history book, it is filled with tragic scenes of people killing each other, and even cannibalism. It truly is "endless joy in fighting among people."
Quoting Professor Ge Jianxiong's "History of Chinese Population Development" from Fudan University: during the chaos at the end of the Western Han Dynasty and the Xin Mang period, the population decreased from 60 million to 35 million; during the chaos at the end of the Eastern Han Dynasty, the population further decreased from 60 million to 23 million; during the chaos at the end of the Sui and Tang Dynasties, the population decreased from 60 million to 25 million; after the An Lushan Rebellion until the Five Dynasties, continuous wars caused the population to decrease from 70 million to 35.4 million in the early Northern Song Dynasty; during the chaos of the Song and Yuan Dynasties, the population of China (referring to the total of the Song, Liao, Jin, and Xia territories) decreased from 145 million to 75 million; during the transition from the Yuan to the Ming Dynasty, the population decreased from 90 million to 71.6 million; during the chaos at the end of the Ming Dynasty, the population decreased from nearly 200 million to 150 million, and the war losses from the Shunzhi to the Kangxi period of the early Qing Dynasty were approximately 20 million. The chaos centered around the Taiping Heavenly Kingdom War caused the population to decrease from 436 million to 364 million in the late Qing Dynasty.
From this, it can be seen that ancient China was never hesitant to harm (kill) its own people, nor did it ever show mercy.
There is a pattern here: before the Yuan Dynasty, the population decreased significantly during the change of dynasties, often by more than half. After the Yuan Dynasty, however, it was not as exaggerated. The author's speculation is that, on the one hand, the population base became larger, and on the other hand, there were multiple densely populated areas with a wider distribution.
I think this makes a lot of sense. With economic development and the shift of economic centers, the population is distributed in multiple regions, making it less easy to be "annihilated" as before in the event of war.
It can be seen that centralization is dangerous, while a federal and decentralized system is safer.
In fact, this point is also reflected in other aspects. For example, during the Spring and Autumn and Warring States periods, various schools of thought were able to engage in vigorous debates, but after the unification of Qin and Han, this prosperity disappeared. Another example is mentioned by He Zhaowu in his book "Touching the Soul of the Era: He Zhaowu Talks about Reading." He mentioned the indoctrination of ideology, which did not exist during the warlord period. The major schools of thought were able to have a relatively open environment for debate, and even arguments. With the unification of the country, arguments disappeared, and only plagiarism remained.