The Expansion of the East Asian World: the Steppe, Central Asia and the South
The third major development under the Han was the large-scale geographic redefinition of the limits of the East Asian world, of which one aspect was the incorporation of the northern peoples described earlier.
In addition to drawing steppe peoples into their state for military purposes, the Han opened relations with the oasis states of Central Asia as part of their wars against the Xiongnu. These states temporarily became Han protectorates, and although this political incursion into what is now Xinjiang was temporary, the area remained part of the Han cultural sphere until the eighth century, when it was swallowed up by the expanding power of Islam. Related to this expansion into Central Asia was the introduction into China in the first centuries ce of Buddhism, which led to Chinese awareness of and travels to what is now Afghanistan and India. This marked a permanent linking of the East Asian cultural world with both Central and South Asia.Less important during the Han dynasty, but more significant for the long-term evolution of empires in East Asia, was the great, southward migration of the peasantry that began in Han times. The Qin and Han armies had conquered the Yangzi River Valley, and pushed south as far as modern Guangzhou. Peasant families began to move south into the Yangzi region during this period under pressure from excess population, the flooding of the Yellow River in the alluvial plain, and the depredations of the nomads in the northwest, who were often resettled inside Han borders to facilitate their employment in the army. This process was accelerated when independent kingdoms were established in Sichuan and the Lower Yangzi when the Han fell in the third century, and it increased further in the fourth century, when the Yellow River Valley became the scene of constant warfare between rival nomad states.
Down through the Northern and Southern Dynasties, until China was reunited at the end of the seventh century, the northern half was ruled by non-Chinese peoples, creating constant pressure for southward migration, a push which was complemented by the pull of the regular rainfall and fertile soils of the south.At the time of reunification of the East Asian world in 589 ce, roughly 40 percent of the registered population of the empire lived in the Yangzi Valley, and in the course of the Tang dynasty (seventh, eighth, and ninth centuries) the erstwhile southern frontier zone became the demographic, economic, and cultural center of China. As late as the middle of the eighth century more than half the total population still lived in the north, but by the end of the thirteenth, only about 15 percent did so, and this was not due to a decline in the north, where the population increased, but the dramatic rise in the south. Although the newly united empire in the sixth century sought to facilitate the binding together of north and south through the creation of the greatest artificial waterway in the history of mankind, the Grand Canal, the two regions remained distinct throughout the history of imperial East Asia.[598]