<P> In 221 BC, the state of Qin completed its conquest over the other Warring States and united China under Qin Shi Huang, the first emperor of China . These conquests, combined with the Legalist reforms started by Shang Yang in the 4th century BC, transformed China from a loose confederation of feudal states to an authoritarian empire . With the transformation, Qin became able to command a far greater assembly of labourers to be used in public works than the prior feudal kingdoms . Also, once unification was achieved, Qin found itself in possession of a large professional army with no more internal enemies to fight and thus had to find a new use for them . Soon after the conquests, in the year 215 BC, the emperor sent the famed general Meng Tian to the Ordos region to drive out the Xiongnu nomads settled there, who had risen from beyond the fallen marginal states along the northern frontier . Qin's campaign against the Xiongnu was preemptive in nature, since there was no pressing nomadic menace to be faced at the time; its aim was to annexe the ambiguous territories of the Ordos and to clearly define the Qin's northern borders . Once the Xiongnu were chased away, Meng Tian introduced 30,000 settler families to colonize the newly conquered territories . </P> <P> Wall configurations were changed to reflect the new borders under the Qin . General Meng Tian erected walls beyond the northern loop of the Yellow River, effectively linking the border walls of Qin, Zhao, and Yan . Concurrent to the building of the frontier wall was the destruction of the walls within China that used to divide one warring state from another--contrary to the outer walls, which were built to stabilize the newly united China, the inner walls threatened the unity of the empire . In the following year, 214 BC, Qin Shi Huang ordered new fortifications to be built along the Yellow River to the west of the Ordos while work continued in the north . This work was completed probably by 212 BC, signalled by Qin Shi Huang's imperial tour of inspection and the construction of the Direct Road (直道) connecting the capital Xianyang with the Ordos . The result was a series of long walls running from Gansu to the seacoast in Manchuria . </P> <P> Details of the construction were not found in the official histories, but it could be inferred that the construction conditions were made especially difficult by the long stretches of mountains and semi-desert that the Great Wall traversed, the sparse populations of these areas, and the frigid winter climate . Although the walls were rammed earth, so the bulk of the building material could be found in situ, transportation of additional supplies and labour remained difficult for the reasons named above . The sinologist Derk Bodde posits in The Cambridge History of China that "for every man whom Meng Tian could put to work at the scene of actual construction, dozens must have been needed to build approaching roads and to transport supplies ." This is supported by the Han dynasty statesman Zhufu Yan's description of Qin Shi Huang's Ordos project in 128 BC: </P> <P>... the land was brackish and arid, crops could not be grown on them...At the time, the young men being drafted were forced to haul boats and barges loaded with baggage trains upstream to sustain a steady supply of food and fodder to the front...Commencing at the departure point a man and his animal could carry thirty zhong (about 176 kilograms (388 lb)) of food supply, by the time they arrived at the destination, they merely delivered one dan (about 29 kilograms (64 lb)) of supply...When the populace had become tired and weary they started to dissipate and abscond . The orphans, the frail, the widowed and the seniors were desperately trying to escape from their appallingly derelict state and died on the wayside as they wandered away from their home . People started to revolt . </P>

How long did it took to build the great wall of china