In Chapter 16 of the book Guns, Germs, and Steel, how, according to Jared Diamond, did the Chinese develop and maintain Sinification?
Diamond portrays the relative cultural homogenization in China as almost without parallel around the world. "Sinification," he says, "involved the drastic homogenization of a huge region in an ancient melting pot," and indeed this process "offers the key to the history of all of East Asia" (324). It took place, for one thing, because China has some geographic characteristics that encouraged cultural unity. Long rivers that ran from east to west, like the Yangtze and Yellow, meant that similar crops were grown along the coast and far inland. Nor is it divided by a major desert. So when northern Chinese peoples developed what might be called advanced civilization, they spread their culture southward--it was their form of writing, for example, that caught on throughout China, because it had no competitors. Indeed, Chinese culture was heavily influential in the Korean Peninsula and Japan as well as Southeast Asia, in addition to what is today the nation-state of China. As Diamond puts it:
Within East Asia, China's head start in food production, technology, writing, and state formation had the consequence that Chinese innovations also contributed heavily to developments in neighboring regions. (332)
Within China itself, the ruling elites were deliberate in their promotion of Chinese culture and writing, taking sometimes brutal measures to stamp out what they regarded as the primitive culture of people in South China. Because of all these factors, Diamond tells us, the "Sinification" of East Asia was really complete by 100 B.C.E.