For the sixth century Central Asia has formed the border between the Russian/Soviet empire, the Islamic world, and the Chinese sphere. It has been a plural society, with a Russian or Russianized elite ruling over Turks and Iranians, a population of Christians, Socialists and Muslims. From the ninth century the Turks also began to enter Transoxiana and to acquire power even within the sedentary societies of the Middle East. The interaction between the two lifestyles and populations—nomad and sedentary, Turkic and Iranian—dominated the history of Central Asia well into the nineteenth century. By the nineteenth century large numbers of sedentarized Turks had joined this group, and most had become either Turkic-speaking or bilingual. With the decline in government tolerance of Islam in the late 1920s, the center increasingly promoted secular traditions for Central Asians; each republic required a set of national characteristics, a literature, a defined historical experience, and a vision of itself in the world.