"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."

what development in the period circa 1250-1450 represents a situation that is the opposite of nomadic conquerors assimilating into the cultures of the conquered societies