Well, let me start by saying that, as I sit here in an at-least-nominally-republican society, typing away in a Latin script, and using a language deeply infused with Romantic vocabulary, and speaking from a high Judeo-Christian social perspective… that reports of fall of Roman Civilization remain somewhat exaggerated.

That said, your point is well-taken… certainly something happened around 476 CE… and that something tends to be viewed, as Gibbon put it, as the Decline and Fall of the Roman Empire…Just as long as we don't look too far to the east, and as long as we say that Merovingians don’t count because of… reasons (and I’m sure there are some very lovely ones that my Roman colleagues can fill us in on!)

Anyways, let’s trend toward the meat of the question: did a similar process happen in China during its tremendously long periods of internecine conflict? You’ve asked the question in 4 parts, and about 3 broad periods (between Zhou and Qin, Han and Sui, and Tang and Song, respectively). These, I’d say are very good choices, since they certainly account for the longest of the struggles to reclaim imperium, and thus the periods that seem the ripest for total social collapse…

But before getting into the specifics, it’s important to note a critical difference in worldview between East and West that will help to explain the differences in both expectation and outcome of the Chinese conflicts versus the Roman “collapse”. There are two main ideas that it’s important to understand: the Mandate of Heaven, and the Dynastic Cycle. The Mandate of Heaven corresponds broadly with the European Divine Right of Kings – with one key difference…its innate transferability. “Created” by the first Kings of Western Zhou after the Duke of Zhou’s victory over the last, corrupted King of Shang, Di Xin. The giant question mark hanging over the victory, however, was “how in the world is what we just did legitimate? How could we possibly be in the right if we just attacked Heaven’s very emissary on Earth?” In a debate between the second Zhou king, Cheng, and his two top advisors, his uncle the Duke Dan of Zhou and Duke Shi of Shao (two of the three Grand Guardians second only to the King himself), King Cheng stated:

Stop! I, the young son [of King Wu], do not dare to disregard the command of the Lord on High. Heaven was beneficent to King Wen [his grandfather], raising up our little country of Zhou, and it was turtle-shell divination that King Wen used, succeeding to receive the mandate […] Wuhu! Heaven in brightly awesome – it helps our grand foundation! […] It is that I will proceed through the borders and lands pointed out by our ancestors; how much more so now that the divination is also auspicious. As so, expansively I will take you east to campaign. Heaven’s mandate is not to be presumed upon; the divination is aligned like this.

Though the specific “role” of Heaven was questioned – notably be the Duke of Zhou, who was a big proponent of the idea that “Heaven helps those who help themselves” – nevertheless the idea would stick and stick hard: the ruler has the divine right to rule, but not based on blood or ancestry, but rather his righteousness… and should the ruler – no matter how ancient or auspicious his line might be – fail to uphold that righteousness, then Heaven could and would strip him of his right to rule and bestow it upon another who would go on to overthrow the unworthy monarch. It’s particularly ingenious from the perspective of a warlord fresh off of victory, in that it both retroactively justifies the already accomplished rebellion, while also throwing up a firewall against any future rebellions. After all, the winner of any such conflict is Heaven’s will – and the loser is in rebellion against Heaven’s Own Son.

The other major concept that traces itself way all through Chinese history is the innate circularity of time itself – as true detective Rust Cohle put it, “Time is a flat circle.” And while any number of other civilizations like to look back and emulate their civilizational predecessors… China tended to view itself particularly strongly through that lens – or as I like to call it, their “Zhou-colored glasses.” Everything’s about getting back to the Zhou Period, which would post-Warring States come to be viewed as the Great Civilizational High Point when everything was great, all the women were strong, all the men were good looking, and all the children were above average. Even as late as the mid 8th century CE, there were rulers like Empress Wu Zetian who’d go whole-hog and actually rename her period of rule back to Zhou in an attempt at emulating “the good ol’ days” from 1800 years prior. This also played out in the circular nature – or rather, the circular understanding - of government, namely in the concept of the “dynastic cycle”, which dominated Chinese political and historical opinions for much of its historiography. Simply put it’s the idea that a dynasty rises, comes to power, improves the nation, slowly declines, causes Heaven to become displeased, loses the Mandate of Heaven, has rebellions crop up against them, and is at last overthrown and replaced with a new dynastic order that has received the Mandate afresh… wash, rinse, repeat ad infinitum. Now there are quite a few issues with that understanding, but the fact that most people in positions of power understood it as true deeply affected how periods of instability affected the empire. To wit – largely the role of a rebel leader was not to try to break away from imperial rule* (*exceptions do apply, please see store for details), but rather to supplant it. Thus even rebellion and ages of civil strife could be incorporated into that cycle-structure of historicity… as the old saying went, “China long united must divide; long divided, must unite.” Though there would always be a drive to shatter an aging and corrupted empire into is constituent parts, there was an equally-strong gravitational pull that would ultimately yank them back together: the understanding that no mandate to rule could be effectively claimed if the realm remained incomplete – one had to prove his worth through overwhelming conquest or not at all (as several of the northern emperors during the 5 Dynasties period found out to their chagrin… one does not simply claim the Mandate of Heaven by calling a foreign king “daddy.” It is folly.).

In terms of population, such conflicts generally weren’t nearly so destructive as to greatly reduce China’s numbers and in fact, as per R. Eno, “During the Warring States years, the overall population of China grew rapidly, spurred by great strides in agricultural technology.” This is not the case during the 3/16 Kingdoms, nor the 5 Dynasties eras, while did see population fall off significantly from their dynastic highs… though typically that was more to do with things like natural disasters like famines and floods, rather than direct conflict.

In terms of the cities, they did not empty during any of these conflicts like we find at the end of the Western Roman Empire, largely because they had – unlike their far-western counterparts – specifically designed their cities to be the safest possible places in the even of attack. Though certainly specific cities were largely abandoned ahead of attacks, there was not the largescale shift to a manoral system because there was no need for one – cities and their massive walls provided far greater defenses than any countryside castle could. In fact, the period that seem most like the “fall of Rome” in the 5th century is actually one of the failed rebellions against the throne – the An Lushan Rebellion from 755-763 CE, which did see an absolutely tremendous fall off in terms of population. This was due to both the destructive nature of the conflict, but at least as much as that, people just up and saying “peace-out” to wherever they’d live until that point, and dropping off the books. In fact, between 755 and 764 – the two censuses that surround the rebellion – the taxpaying population went from 52 million in 8.9 million households, to just 16.9 million in 2.9 million households. Even so, the idea of city = safety would never really lose its luster for China.