<P> In 1368, the Hongwu Emperor (Zhu Yuanzhang, r . 1368--98) ousted the Mongol - led Yuan dynasty from China to inaugurate the Ming dynasty . The Mongols fled back to Mongolia, but even after numerous campaigns, the Mongol problem remained . </P> <P> During his early reign, Hongwu set up the "eight outer garrisons" close to the steppe and an inner line of forts more suitable for defence . The inner line was the forerunner to the Ming Great Wall . In 1373, as Ming forces encountered setbacks, Hongwu put more emphasis on defence and adopted Hua Yunlong's (華 雲龍) suggestion to establish garrisons at 130 passes and other strategic points in the Beijing area . More positions were set up in the years up Hongwu's death in 1398, and watchtowers were manned from the Bohai Sea to Beijing and further onto the Mongolian steppes . These positions, however, were not for a linear defence but rather a regional one in which walls did not feature heavily, and offensive tactics remained the overarching policy at the time . In 1421, the Ming capital was relocated from Nanjing in the south to Beijing in the north, partly to better manage the Mongol situation . Thus defenses were concentrated around Beijing, where stone and earth began to replace rammed earth in strategic passes . A wall was erected by the Ming in Liaodong to protect Han settlers from a possible threat from the Jurched - Mongol Oriyanghan around 1442 . In 1467--68, expansion of the wall provided further protection for the region from against attacks by the Jianzhou Jurchens in the northeast . </P> <P> Meanwhile, the outer defenses were gradually moved inward, thereby sacrificing a vital foothold in the steppe transitional zone . Despite the withdrawal from the steppe, the Ming military remained in a strong position against the nomads until the Tumu Crisis in 1449, which caused the collapse of the early Ming security system . Over half of the campaigning Chinese army perished in the conflict, while the Mongols captured the Zhengtong Emperor . This military debacle shattered the Chinese military might that had so impressed and given pause to the Mongols since the beginning of the dynasty, and caused the Ming to be on the defensive ever after . </P> <P> The deterioration of the Ming military position in the steppe transitional zone gave rise to nomadic raids into Ming territory, including the crucial Ordos region, on a level unprecedented since the dynasty's founding . After decades of deliberation between an offensive strategy and an accommodative policy, the decision to build the first major Ming walls in the Ordos was agreed upon as an acceptable compromise the 1470s . </P>

When did construction on the great wall of china begin