<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> <P> Xinjiang, considered part of the Turkestan region, consists of an amalgamation of deserts, oases, and dry steppe barely suitable for agriculture . When influence from the steppe powers of Mongolia waned, the various Central Asian oasis kingdoms and nomadic clans like the Göktürks and Uyghurs were able to form their own states and confederations that threatened China at times . China proper is connected to this area by the Hexi Corridor, a narrow string of oases bounded by the Gobi Desert to the north and the high Tibetan Plateau to the south . In addition to considerations of frontier defence, the Hexi Corridor also formed an important part of the Silk Road trade route . Thus it was also in China's economic interest to control this stretch of land, and hence the Great Wall's western terminus is in this corridor--the Yumen Pass during Han times and the Jiayu Pass during the Ming dynasty and thereafter . </P> <P> One of the first mentions of a wall built against northern invaders is found in a poem, dated from the seventh century BC, recorded in the Classic of Poetry . The poem tells of a king, now identified as King Xuan (r . 827--782 BC) of the Western Zhou dynasty (1046--771 BC), who commanded General Nan Zhong (南仲) to build a wall in the northern regions to fend off the Xianyun . The Xianyun, whose base of power was in the Ordos region, were regarded as part of the charioteering Rong tribes, and their attacks aimed at the early Zhou capital region of Haojing were probably the reason for King Xuan's response . Nan Zhong's campaign was recorded as a great victory . However, only a few years later in 771 BC another branch of the Rong people, the Quanrong, responded to a summons by the renegade Marquess of Shen by over-running the Zhou defences and laying waste to the capital . The cataclysmic event killed King Xuan's successor King You (795--771 BC), forced the court to move the capital east to Chengzhou (成 周, later known as Luoyang) a year later, and thus ushered in the Eastern Zhou dynasty (770--256 BC). Most importantly, the fall of Western Zhou redistributed power to the states that had acknowledged Zhou's nominal rulership . The rule of the Eastern Zhou dynasty was marked by bloody interstate anarchy . With smaller states being annexed and larger states waging constant war upon one another, many rulers came to feel the need to erect walls to protect their borders . Of the earliest textual reference to such a wall was the State of Chu's wall of 656 BC, 1,400 metres (4,600 feet) of which were excavated in southern Henan province in the modern era . The State of Qi also had fortified borders up by the 7th century BC, and the extant portions in Shandong province had been christened the Great Wall of Qi . The State of Wei built two walls, the western one completed in 361 BC and the eastern in 356 BC, with the extant western wall found in Hancheng, Shaanxi . Even non-Chinese peoples built walls, such as the Di state of Zhongshan and the Yiqu Rong (義 渠), whose walls were intended to defend against the State of Qin . </P> <P> Of these walls, those of the northern states Yan, Zhao, and Qin were connected by Qin Shi Huang when he united the Chinese states in 221 BC . </P>

When did the construction of the great wall of china start
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