"Throughout its history, Central Asia has provided the ancient civilized empires on its borders with new Shahs, Sultans, or Sons of Heaven. These periodic invasions by the nomads of the steppe, whose khans ascended the thrones of Changan, Luoyang, Kaifeng, or Beijing*, of Isfahan or Tabriz**, Delhi or Constantinople, became one of the geographic laws of history. But there was another, opposing law which brought about the slow absorption of the invaders by the ancient civilized lands. The civilizations of China and Persia, though conquered, would in the long run vanquish their conquerors, intoxicating them with the pleasures of settled life, lulling them to sleep, and assimilating them culturally. Often, only fifty years after a conquest, the culturally Sinicized or Persianized former barbarian would be the first to stand guard over his adopted civilization and protect it against fresh nomadic onslaughts." *capital cities of various Chinese dynasties **capital cities of various Persian dynasties René Grousset, French historian of Central Asia, The Empire of the Steppes: A History of Central Asia, 1939 The general pattern of nomadic conquest followed by the assimilation of the conquerors into the cultures of the conquered societies was most clearly expressed in which of the developments comprises in the period circa 1250–1450 ?