[h4nkoslo]

To describe China as "unified" ignores their dozens of civil wars and conquests by external groups. They kind of have an at best metastable "tough shell / gooey center" dynamic, and I would not be surprised if their proportional deaths by strife were greater than the European number over most timeframes.

[dredmorbius]

Europe was settled and civilised relatively lately (at least by h. sapiens), and saw constant assault from various quarters. The critical region was the Mediterranian, which saw the Levant, Mesopotamia, Egypt, Cretian and Minoan civilisations, the maritime culture of the Phoenecians (and perhaps an earlier one, I'm a tad rusty), Greece, Macedonia, Persia, the Romans, Gauls, Huns, Carthage, Moors, Norsemen / Normans, Anglos and Saxons, etc., etc. There was no cultural or linguistic unity among most of those.

European territory made external travel (around the coastline) quite easy (good for invaders, hard for defenders to move to where the attackers are).

China's geography had a much smaller coastline (relative to landmass), and a far superior interior river network, augmented early (~1,000 BCE IIRC) with manmade canals, making internal movement of forces far easier. China also wasn't surrounded by capable enemies -- India was the nearest significant civilisation. The Mongols managed to invade, yes, but they were a fluke and also managed to occupy nearly all the Eurasian landmass, from the Pacific to central Europe and India. (The Mughal Empire was the result of Mongol invaders of India.)

China did see internal strife, but those splits resolved with a resumption of the previous Chinese bureaucracy rather than a replacement of it, as seemed more typical of Europe.