Sure, in all cases China has had to be reunified by force, but the very fact that it was possible to use military force to achieve a lasting political unification speaks of an underlying social unity. It is almost impossible to find an example anywhere in history of any kind of shift in borders happening without military force being used. This is just the nature of human societies. Conquest is a means to an end.
There were only... four times that there was an actual reunification above a brief civil war. The Qin, the Jin, the Sui, and the Song. The rest were either brief civil wars between maybe two or three factions while or just regime changes. The Ming showed up a general revolt from the Yuan Dynasty, while there was some infighting it was quickly decided the Ming would rule. I'm tempted to say that Yuan-Ming-Qing were all quick wars, because they were all done in under a decade with only two opposing sides bashing each other's heads in.
And what should also be noted is that the Qin was pitifully small at the time with only lose control over what it conquered. The brief war between the Han and Chu ended with the Han expanding things greatly. The Jin conquered just one foe, Wu, and it was mostly culturally motivated. The Sui showed up after 300 years of civil war, but then again it was a period known as the Norther and Southern era of China, there were only two kingdoms at any one time, again apart from brief civil wars inside individual Kingdoms. The Song was a true... well, again what it had was relatively small. From there it expanded against the Wu Hu Kingdoms only to be forced to a Northern Southern analog.
The borders of what we know of as China was never reached until the Qing. So if we were to just focus on the four times it had to be reunited and just not a brief civil war lasting no more than a decade or two. If we look at Persia, the Steppes, or India we find
very similar occurrences.
In China, it has been possible time and again for strong warlords and kings to unite the country through military force, and the country has remained united until the inevitable erosion of their short-lived state structures ushers in another cycle of political collapse. This has never deterred Chinese civilization from being able to consistently reunite under a single imperial banner.
Four times, only twice were what I'd call real triumphs and the Song and Qin were very small. In a sense it would be uniting an area the size of, well, Germany and Poland with maybe some added Belarus. It was after this unity that they would expand. Most maps you see of the Song or Qin is at its highest extent, never what it was when unity first came.
As for Europe being united twice, we have three examples that are similar. We have the Roman Empire, the Carolingian Empire, and the Holy Roman Empire. Each of these united an area greater to, or about the same, the Chinese points of unity. And all the time they had powerful foreign Kingdoms on the borders. The
Xiongnu,
Jurchen,
Nanchao,
Northern Yuan,
Tibet,
Wuhuan,
Xianbei,
Northern Wei, and many, many, others.
Furthermore, the dominance of the Roman Empire was only possible because the European economy was so heavily unbalanced in favor of the Mediterranean - Northern and Western Europe were still in their economic infancy and essentially colonies of Rome. The great achievement of the Roman Empire was not in uniting Europe but in uniting the Greek and Roman worlds - the only parts of Europe that mattered in those days. If the regions of Europe had been more equal in power the empire would have been an impossible proposition.
It was much less balanced in the far East, trust me on that. China was the center of economic power only four times. The Han (where it was still bested by the Xiongnu), The Tang (which collapsed due to pressure from "barbarians", but made the notable campaign of fighting against the Caliphate and losing badly to Tibet), The Song (and most of that power was actually under the Jin, the Song got some bare necessities), and the era of the Ming-Qing. And those eras of being the most unbalanced were actually still much more balanced than most other times. It would be under the Ming-Qing where China was a superpower until the mid1800s.