<P> According to the model by sinologist Karl August Wittfogel, the loess soils of Shaanxi made it possible for the Chinese to develop irrigated agriculture early on . Although this allowed them to expand into the lower reaches of the Yellow River valley, such extensive waterworks on an ever - increasing scale required collective labour, something that could only be managed by some form of bureaucracy . Thus the scholar - bureaucrats came to the fore to keep track of the income and expenses of the granaries . Walled cities grew up around the granaries for reasons of defence along with ease of administration; they kept invaders out and ensured that citizens remained within . These cities combined to become feudal states, which eventually united to become an empire . Likewise, according to this model, walls not only enveloped cities as time went by, but also lined the borders of the feudal states and eventually the whole Chinese empire to provide protection against raids from the agrarian northern steppes . </P> <P> The steppe societies of Inner Asia, whose climate favoured a pastoral economy, stood in stark contrast to the Chinese mode of development . As animal herds are migratory by nature, communities could not afford to be stationary and therefore evolved as nomads . According to the influential Mongolist Owen Lattimore this lifestyle proved to be incompatible with the Chinese economic model . As the steppe population grew, pastoral agriculture alone could not support the population, and tribal alliances needed to be maintained by material rewards . For these needs, the nomads had to turn to the settled societies to get grains, metal tools, and luxury goods, which they could not produce by themselves . If denied trade by the settled peoples, the nomads would resort to raiding or even conquest . </P> <P> Potential nomadic incursion from three main areas of Inner Asia caused concern to northern China: Mongolia to the north, Manchuria to the northeast, and Xinjiang to the northwest . Of the three, China's chief concern since the earliest times had been Mongolia--the home of many of the country's fiercest enemies including the Xiongnu, the Xianbei, the Khitans, and the Mongols . The Gobi Desert, which accounts for two - thirds of Mongolia's area, divided the main northern and southern grazing lands and pushed the pastoral nomads to the fringes of the steppe . On the southern side (Inner Mongolia), this pressure brought the nomads into contact with China . </P> <P> For the most part, barring intermittent passes and valleys (the major one being the corridor through Zhangjiakou and the Juyong Pass), the North China Plain remained shielded from the Mongolian steppe by the Yin Mountains . However, if this defence were breached, China's flat terrain offered no protection to the cities on the plain, including the imperial capitals of Beijing, Kaifeng, and Luoyang . Heading west along the Yin Mountains, the range ends where the Yellow River circles northwards upstream in the area known as the Ordos Loop--technically part of the steppe, but capable of irrigated agriculture . Although the Yellow River formed a theoretical natural boundary with the north, such a border so far into the steppe was difficult to maintain . The lands south of the Yellow River--the Hetao, the Ordos Desert, and the Loess Plateau--provided no natural barriers on the approach to the Wei River valley, the oft - called cradle of Chinese civilization where the ancient capital Xi'an lay . As such, control of the Ordos remained extremely important for the rulers of China: not only for potential influence over the steppe, but also for the security of China proper . The region's strategic importance combined with its untenability led many dynasties to place their first walls here . </P>

How did the great wall of china get destroyed