From Yangtze: Nature, History, and the River, by Lyman P. Van Slyke (Stanford Alumni Assn., 1988), p. 50:
Unlike Europe, where the Holy Roman Empire was but a faint echo of Rome’s glory, China’s medieval period was one of the most powerful and resplendent eras of her history. Once again a short unifying dynasty (the Sui) was followed by a powerful and long-lived inheritor the T’ang. In many ways, the T’ang was even more glorious an era than the Han had been, but after roughly two centuries, its lustre was rapidly dimming. By the end of the T’ang, the frontier marches were virtually independent, and the succeeding Sung dynasty was never able to solve its border problems. In the twelfth century, frontier defenses collapsed. North China fell once again into the hands of barbarians, this time from the region we today call Manchuria, and once again a refugee Chinese regime the Southern Sung was driven south to the lower Yangtze, the lands of Ch’u and Wu, where it made its “temporary” capital in the lovely city of Hangchow.
After the Southern Sung the late empire there were interregna but never again a long period of disunity. Ironically, however, this unity was twice imposed by non-Chinese peoples from beyond the Wall: the Mongols of Genghis and Kubla Khan, to whose court came Marco Polo; and then the last dynasty, and in some ways the culmination of the imperial process, the Manchu Ch’ing dynasty, which occupied the Dragon Throne from 1644 until 1911, when in one of history’s greatest not-with-a-bang anticlimaxes the abdication of a six-year-old boy emperor ended not only that dynasty but all dynasties, and more than 2,000 years of imperial history came to an end. These two alien dynasties the Yuan (1279-1368) and the Ch’ing (1644-1911) were the only two periods of foreign rule over all of China.
This very brief chronological survey introduces the names of the principal dynastic periods, and provides a reference chart, since I will be moving back and forth in time through most of the essays in this book. Periods of disunity also influence the story directly, since it was under the impact of barbarian onslaughts that Chinese influence was pushed into the Yangtze Valley more rapidly and forcibly than would otherwise have been the case. Indeed, during the three and a half centuries between the Han and the T’ang, refugee populations and exile dynasties so developed the lower basins of the Long River that by the time of the Sui-T’ang unification this region had replaced the North China Plain as the economic center, the grain basket, of China. A similar developmental pulsation took place again during the Southern Sung dynasty approximately A.D.1100 to 1300 when the north was lost to the Chin, before both were overwhelmed by the Mongol hordes of Genghis Khan and his grandson, Kubla.