It's a rather complicated question to answer because China did not have a clear concept of nationalism (as defined by Benedict Anderson) before 1949.

Many of the subjects in an empire would identify themselves according to the ruling dynasty, so a subject of the Qin empire would be a "Qin" person, or a subject of the Han empire would be a "Han". (The latter would be the basis for the Chinese ethnic majority identifying as Han. Also, many Chinese today still refer to themselves as Tang people in identification with the Tang Dynasty. This is why they still refer to Chinatown as Tang Ren Jie.)

There is also the matter of shifting boundaries. Conflicts with other tribes and kingdoms meant that each empire's borders would shift constantly. The territory of Qin China, for example, was mostly concentrated near the Yellow River. It is very much different from the territory of China during the height of Tang power, or present-day China's boundaries.

So how did the concept of a unified China survive? Primarily due to the socio-political groundwork laid down by Qin and Han. Please take a look at this very helpful collection, the History of Imperial China for more information.

Also, it should be noted that the concept of the "Mandate of Heaven" and the "Son of Heaven" made it possible to create a sense of continuity despite the change in leadership. Confucianism (rooted in Zhou and possibly pre-Zhou social mores and political principles) provides legitimacy through its concept of the Mandate of Heaven, wherein a bad ruler can be replaced by someone more deserving who has earned Heaven's approval.

Finally, I second /u/skyanvil on the matter of a shared cultural/social identity. Despite not having a clear concept of nationalism, the presence of shared history, culture, language, and customs made China's mostly-unified 5000 years possible.

It seems to me that there is a pretty big part of this question that has not yet been discussed. In Europe, the collapse of the Western regime was not merely a political collapse, but coincided with a general collapse of urban civilization. In the 8th century in Europe, urban populations were considerably lower, as was the general population, than during Roman times. The economy of Europe transformed from one interconnected and protected by Roman political power to one predominantly defined by local forms of production centered on the manor. For one, the overwhelmingly local character of Europe likely contributed to the development of many localized customs and ethno-linguistic identities that perhaps might have made political unification more difficult. On the other hand, and I think more importantly, the construction and maintenance of a large, complex political formation was made more difficult by the generally depressed and simple nature of the European economy. By the time of European economic 'recovery,' there were too many disparate political powers in Europe competing with one another to get an edge with obviously the closest being Napoleon's France for a very brief moment.

By contrast, to the extent that the end of the Han and the invasions and warfare of the Northern/Southern period disrupted commerce and manufacture, it was not nearly to the extent of the European cataclysm. By the 8th century, again, Chang'an (modern day Xian, a center of political power as early as the Zhou state and the main capital of the Tang dynasty) had a population (including suburbs) of perhaps 1.9 million people and was probably the largest and most sophisticated urban settlement in human history up to that point. Chang'an was also essentially the beginning or the end of the Silk Road, depending on one's perspective, and had an incredibly diverse and cosmopolitan character, as did many of the other major cities in the Tang period - Luoyang certainly as the other seat of political power, but also commercial cities like Jiangdu, Suzhou, and Hangzhou (the later capital of the southern Song). Europe, by contrast, had almost no urban activity in comparison. The strength and continued functioning of the Chinese economy allowed for the continuation of high urban populations and for the continued existence of the shih as a force of governmental and administrative power. It also allowed for the resources to concentrate sufficiently to establish and maintain vast political systems. These systems were obviously not perfect. Tang power was great, but just 150 years after Li Yuan's founding of the dynasty, Chang'an lost practical control of large portions of the northeast and Chinese civilization was wracked by warfare and destruction as part of An Lushan's rebellion. The Tang's eventual collapse in 907 was prefigured by many decades by the collapse of central political power and the dominance of warlords and bureaucratic administrators of state monopolies.

Ultimately there are many reasons to think that a unified Europe could have been possible despite differences between peoples at various times, but I think the chief problem was an issue not of consciousness but of practical and material limitations to political power.

I would like to present slightly diffrent version than /u/bokchoybaby and /u/skyanvil.

First of all we can see some parallels between Roman/European and Chinese history. One of them, which is in my opinion most important, is the fact, that both countries were in dome point divided into two empires - Rome into Western and Eastern Empire, China would occasionaly split around Yangtze River.

In case of Roman Empire the split was caused by political issues and resulted in deep cultural diffrences, which are visible even today (Schism). China on the other hand would be conquered by a neighbor from North. After the Jurchen Invasion, Chinese of Song dynasty had to flee south from Yangtze, and Lin'an became the new capital. Why this is so important? Up to this point Chinese culture developed north from Yangtze. This situation (elites going south) helped in sinization of those territories (remember that even today ethnic diversification of China is great - diffrences between Mandarin and Cantonese are IMHO bigger than between Polish and Russian). Important to remember is fact, that time of Songs is perceived as a moment when classical Chinese culture was born. Before them we had Tangs, who were a great Steppe-oriented nation, with far more diverse society (both in ethnical and religious dimensions). Creation of a self-aware nation (it also happened in Rome!) was a key to success. During the time of Jurchen (Jin) rule, North maintained its Chinese character. After that we have Mongol episode and another national dynasty, Mings. After them came Qing dynasty, which also after a while became a victim to sinization.

In Europe everything was diffrent - conquest by barbarians happened few hundreds of years earlier. And while East managed to exist through this turmoil, it created very distinct culture than West. Religion is a key to understanding, why there could be no way to unify the Empire - there was no Investiture Controversy in Byzantium, Emperor was above the Church. Political fragmentation of West led to stronger position of the Church, Byzantium was one state, so Emperors had more tools to ensure their influences. Christian idea of separation of Church and State, lessening the importance of State (countries weren't as important as the Church) - those factors weren't present in China, where religion was clearly a state matter and most important ceremonies were held by the Emperor.

Next, we have diversity among barbarians and geography. In China they came from North - Jurchens/Manchurians (that's the same nation!), Mongolians. Even if we count other tribes, like Kitans, they were either Mongolian or Tungusic. From other directions, little to nothing threatened China. Most of South was mountains and/or jungle, in the West they had mountains and deserts. Japan wasn't a threat either (not until 16th century, but invasion was stopped in Korea, which also was fragmented and to little to be a real threat to the Empire). Remember, that a lot of terrains were Chinese only nominally, like Yunnan, which was full of diffrent tribes and was chinese only on paper.

As for Rome - whole Western border was in danger. Either attacked by barbarians, or strong countries like Parthians. We had Goths, Franks, Avars, Magyars, Bulgarians, Slavs, Sarmatians. There was no real border separating them from territories of the Empire - apart from Alps, which were right in the middle, and it wasn't hard to go around them, attacking Greece or Galia. Even after fall of Rome, terrains which were quite unified in religious terms (and this is a huge factor) were not spared by Lady Fortune and Arabs decided that they want to conquer them, thus obtaining large portion of Christianitas.

I'm not going to answer, just take issue with the question. The idea that a unified China "survived" while a unified roman empire didn't discounts the byzantines and Holy Roman Emperors, while elevating a history of diversity, disunity, and frequent dynastic struggle in China to a "surviving idea of unity." The idea of a unified Rome/Europe lasted straight up until Napoleon, who intentionally cast himself in the role of a Roman emperor. If he had won and the 20th century went differently in China, this might be a curious student in from the country of Hunan asking why the unified idea of rome "survived" in Greater France while China developed into many different states.

I think questions like this are often rooted in a complete lack of understanding about the tremendous diversity there is in China, still, despite a program in the 60s designed explicitly to eliminate it, which itself followed a massive nationalization effort by the Nationalist government. To believe that "China" is a monolithic political entity going back for-all-intents-and-purposes forever is to completely take the CCP's propaganda at face value.

Also, anyone who gives you a just-so story of any of these kinds:

• It's just geography!

• Something something china had a unified writing system (because europe didn't have Latin, apparently), something something vernaculars can't develop in China (even though they did)

• Examination system!

• Religions are different!

...is generally wrong. Many of these are interesting concepts and all can tell us interesting things about their respective areas of history, and all are constantly overstated and anachronistically projected across millennia (with analogues in their counterpart ignored) to present a tidy narrative that goes "they are like this and we are like this."

I don't know as much on Chinese history as I would like to do, but I think an important part of the answer may be our distorted view of the succession of Chinese empires, due to our ethnocentric bias (to which we could add the nationalist reconstruction of Chinese history). After all, you could argue that Napoleon's son was entitled “king of Rome” and destined to inherit the greatest part of Europe — doesn't it clearly show that the Roman Empire still existed in 1810? For us, the differences between Iulius Caesar and Napoleon seem obvious; they may not be so blatant for someone without a good knowledge of European history. Conversely, we definitely tend to think that all Chinese (and, more generally, non-European) polities were more similar than they really were. Were the Sui and the Jin less different than Charlemagne and Constantine?

One important thing to remember is that there wasn't a roman empire anymore. There were two. Divided along the lines where Latin was the common language, and Greek. The eastern survived for a good amount of time with a large amount of power.

China unified its writing system of hieroglyphs and ideograms, which created the illusion of a single language. You can't write medieval German in Latin words and spellings.

don't forget that CHina has a ton of dialects that are quite frankly, wholly different languages from Mandarin. By this, I mean one person from one part of China if spoken to in dialect, would not be able to understand the other....they all have the same written language as a bond.

China also had Siberia and Mongolia as a buffer....and Russia. IT also has the mtn's keeping it from India.

For starters, the idea of a large empire in China is older than the Roman civilization. The Zhou stretch back past 1000 BCE.

The brutal unification under Qin unified the written language, which stayed the same for all of China since then. Sure Europe had Latin, but that was the language of a religion which was not accepted by everyone until the 10th century AD. And written Latin could never be written to express ideas in vernacular, while written Chinese could be used for vernacular in most dialects.

China used to be called "中原"... Central Planes. Travel from one part of the land to others were only effectively impeded by two rivers, where-as Europe has large mountains has multiple geographic features that serve to protect separate areas.

Then there is the nature of the nomadic peoples themselves. It took a great deal of resources and politiking to hold the Mongols (and related peoples, ie Jurchens, etc) at bay in the North. This effort required a large unified state. When the efforts failed - in the Yuan and Qing dynasties - the nomads swept in and took over the whole country, so it was in their interest to keep their spoils of war unified.

One final point... China was actually not unified for much of it's history.