It was only after the Hsia empire in North China, and then the empire of the Juchên, had been destroyed by the Mongols, and only after long and remarkably modern tactical preparation, that the Mongols conquered South China, the empire of the Sung dynasty. They were now faced with the problem of ruling their great new empire. The conqueror of that empire, Kublai, himself recognized that China could not be treated in quite the same way as the Mongols' previous conquests; he therefore separated the empire in China from the rest of the Mongol empire. Mongol China became an independent realm within the Mongol empire, a sort of Dominion. The Mongol rulers were well aware that in spite of their numerical strength they were still only a minority in China, and this implied certain dangers. They therefore elaborated a "nationality legislation", the first of its kind in the Far East. The purpose of this legislation was, of course, to be the protection of the Mongols. The population of conquered China was divided into four groups--(1) Mongols, themselves falling into four sub-groups (the oldest Mongol tribes, the White Tatars, the Black Tatars, the Wild Tatars); (2) Central Asian auxiliaries (Naimans, Uighurs, and various other Turkish people, Tanguts, and so on); (3) North Chinese; (4) South Chinese. The Mongols formed the privileged ruling class. They remained militarily organized, and were distributed in garrisons over all the big towns of China as soldiers, maintained by the state. All the higher government posts were reserved for them, so that they also formed the heads of the official staffs. The auxiliary peoples were also admitted into the government service; they, too, had privileges, but were not all soldiers but in many cases merchants, who used their privileged position to promote business. Not a few of these merchants were Uighurs and Mohammedans; many Uighurs were also employed as clerks, as the Mongols were very often unable to read and write Chinese, and the government offices were bilingual, working in Mongolian and Chinese. The clever Uighurs quickly learned enough of both languages for official purposes, and made themselves indispensable assistants to the Mongols. Persian, the main language of administration in the western parts of the Mongol empire besides Uighuric, also was a _lingua franca_ among the new rulers of China.