I can’t remember who was talking and speculating about this, but China had been through a number of centralizing than fracturing periods then centralizing again and repeat (“the Empire long united, must divide. Long divided, must unite”) for whatever reason.
When periods of unification happened, an outside force or theory of government would come and unite all of China (whatever “all” meant for that period). The Yuan dynasty from Mongolia, the Han dynasty bringing about confuscianisn on a grand governmental scale, etc. And when it came time for to divide after disunity and corruption of the Qing in the mid 19th century, it probably would’ve been under the Heavenly Kingdom syncretized Christianity - except for the intervention of now world bestriding imperial European great powers, which had a vested interest in maintaining the Qing and later the Republic.
In the end, it was still an outside force that put an end to banditry, disunity, etc. Or another fracturing period. It was marxism and Maoism, and it was the establishment of the PRC (notwithstanding the ROC in Taiwan).
While this can be a starting point to study and understand Chinese history, it’s still fall into the trap of simplistic generalisation, ignoring unique material conditions at the time.
Using Han Dynasty (漢) as an example. Formed by Liu Bang in 202 BCE, it was a vastly decentralised dynasty compare to it’s predecessor (the Qin, 秦), due to (1) fierce pressure from noble class, (2) concerns of weak imperial army, and (3) the consequences of fall of Qin followed by the struggles between rebels. It took around 50 years for the emperors to successfully take back control from them. This form a material foundation for Emperor Wu to conduct various expeditions to the north to defeat and eventually crush the threat from Xiongnu, which persisted since the Warring States period.
At the same time, the marriages between the imperial family and prominent generals gave rise to a powerful faction, wàiqì (外戚), the consort kin, which its replacement cycle almost akin to the life cycle of the emperor. Exacerbated by shoft-living emperors, the consort kin asserted enormous power over imperial politics, in which its ambitious was finally took over and usurped the power for themselves, establishing the Xin dynasty (新, literally a new dynasty). However, because of it’s unpopularity among other ruling class and peasants, it was finally brought down, and Han dynasty was re-established.
Learning the lesson from such disaster, Han emperors began to give more power to a lesser educated class, the Eunuch. Despite what the propaganda might suggest, this faction, at a later time, will gain more literacy and competency as they had more experience in administrative matters as they got more involved in imperial politics. From here, the struggles between these two factions will only intensified, until they both met their political destruction when both He Jin (何進), the brother of the consort, and the Tens (十常侍), a group of powerful eunuchs were killed. Losing both incredible factions, and with a child emperor, the imperial authority can be said to be nonexistent, only to be used as muppet for those who can control them. Dong Zhuo (董卓) and Cao cao (曹操) were the most prominent.
The political vacuum was soon filled up by the new noble class, the bureaucrats (士族). Despite the name, their powers stemmed from family pretisge, individual prominence, and the most important: being landowner, that is to have slaves and serfs working for their lands. As such, I much prefer to call them as Shizu to eliminate any ambiguity. Tons of prominent people from Han later period were coming from this background, including Yuan Shao (袁紹) from the Runan’s family of Yuan, Cao cao from the family of Xiahou, Xun Yu (荀彧, Cao cao’s administrative and military staff) from the Yinchuan’s family of Xun, Zhuge Liang (諸葛亮) from the Langya’s family of Zhuge, and Sima Yi (司馬懿, the grandfather of the founder of Jin dynasty, Sima Yan) from the Wenxian’s family of Sima. Shizu either recruited peasants and their relatives to fill their rank (like Cao cao and Yuan Shao), or joined other warlords (usually other Shizu) at the time to persue greater opportunities (Xun Yu, Zhuge Liang, and Sima Yi).
You might wonder why I didn’t mention the Yellow Turban Rebellion until now. It is simply because the rebellion, though cause great turbulence, was ultimately crushed by the imperial army, which at that time still command a great authority over other classes. This was before the death of Emperor Ling, long before when the conspiracy to kill the Tens was a thing. An interesting observation from me is that, Shizu was the class that is at the forefront against the rebellion. I assume that is because of the nature of the rebellion being a peasant revolt over increasing land concentration, Shizu was anxious about their policial grip over local community, and rushed to the side of the imperial army, eager to prove their worth. With extreme luck, they successfully subjugated the peasants and commanded great authority due to the collapse of imperial powers. Their power remained unchallenged, and finally usurped the power from Han dynasty, establishing Wei dynasty (family of Cao) and later Jin dynasty (family of Sima).
We can notice that this is not a simple narrative of gradual centralisation of a state. In fact, the Jin dynasty was even more decentralised than Han dynasty. The process of state centralisation was only continued during Tang and Song dynasty, due to the advancement of statecraft and the suppression of other classes. The narrative of “the Empire long united, must divide. Long divided, must united” (from the Romance of the Three Kingdoms) is a superstitious belief that disregards the importance of class struggle. The dynasty name might change, but the reality might not. Reading history with this narrative is simply reading history through the lens of monarchs; it’s Chinese version of Great People Theory.
The fall of Qing Dynasty, was not simply because of the invasion of imperial forces. In fact, Qing always struggle with internal fractures, due to its “Manchu” identity from its imperial family, its Eight Banners (八旗, treat them like military nobles, mostly of Manchu ethnicity but with prominent Liaodong Han minority), and Shizu (mostly of Han ethnicity). The Qing authority never trusted local conscription from Han ethnicity (other than Liaodong), and instead heavily relied on Eight Banners military prowess. As the latter corroded due to rampant corruptions and favouritism, the imperial control over the vast territories of China weaken. Qing didn’t try to concentrate the state power, because that means the introduction of untrusted Han components into imperial politics, which they were paranoid of (past trauma from the Revolt of Three Feudatories). So, you would find out that the First Opium War was fought, not simply between the English and the Qing imperial army, but it was actually a war fought between various local bureaucrats, with an emperor that was gradually losing his control over them. Taiping Rebellion took advantage of this nature, sprung up and rose against the Qing dynasty, resulted in astounding success. To save Qing from utter collapse, the ruling class finally conceded, allowing private army to be conscripted and trained by local Shizu, sealing its final fate of being overthrown by this new political force.
For sure the Taiping rebels may overtake the power from Qing imperial army and built the so called Christianised Kingdom of China. But the one that disrupted such progression is from the intuition of self-preservation from the ruling class. Imperialist force may contribute to such factor, but it was ultimately the decision made by the oppressor, because it is far too dangerous for them to be ruled by the peasants, than by the outsiders.