<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> <P> Although Manchuria is home to the agricultural lands of the Liao River valley, its location beyond the northern mountains relegated it to the relative periphery of Chinese concern . When Chinese state control became weak, at various points in history Manchuria fell under the control of the forest peoples of the area, including the Jurchens and the Manchus . The most crucial route that links Manchuria and the North China Plain is a narrow coastal strip of land, wedged between the Bohai Sea and the Yan Mountains, called the Shanhai Pass (literally the "mountain and sea pass"). The pass gained much importance during the later dynasties, when the capital was set in Beijing, a mere 300 kilometres (190 miles) away . In addition to the Shanhai Pass, a handful of mountain passes also provide access from Manchuria into China through the Yan Mountains, chief among them the Gubeikou and Xifengkou (Chinese: 喜 峰 口). </P>

When and how was the great wall of china built