<P> The campaigns against the Xiongnu and other nomadic peoples of the west exhausted the imperial treasury, and the expansionist policies were reverted in favour of peace under Emperor Wu's successors . The peace was largely respected even when the Han throne was usurped by the minister Wang Mang in 9 AD, beginning a brief 15 - year interregnum known as the Xin dynasty (9--23). Despite high tensions between the Xin and the Xiongnu resulting in the deployment of 300,000 men on the Great Wall, no major fighting broke out beyond minor raids . Instead, popular discontent led to banditry and, ultimately, full - scale rebellion . The civil war ended with the Liu clan on the throne again, beginning the Eastern Han dynasty (25--220). </P> <P> The restorer Emperor Guangwu (r . 25--57 AD) initiated several projects to consolidate his control within the frontier regions . Defense works were established to the east of the Yanmen Pass, with a line of fortifications and beacon fires stretching from Pingcheng County (present - day Datong) through the valley of the Sanggan River to Dai County, Shanxi . By 38 AD, as a result of raids by the Xiongnu further to the west against the Wei River valley, orders were given for a series of walls to be constructed as defences for the Fen River, the southward course of the Yellow River, and the region of the former imperial capital, Chang'an . These constructions were defensive in nature, which marked a shift from the offensive walls of the preceding Emperor Wu and the rulers of the Warring States . By the early 40s AD the northern frontiers of China had undergone drastic change: the line of the imperial frontier followed not the advanced positions conquered by Emperor Wu but the rear defences indicated roughly by the modern (Ming dynasty) Great Wall . The Ordos region, northern Shanxi, and the upper Luan River basin around Chengde were abandoned and left to the control of the Xiongnu . The rest of the frontier remained somewhat intact until the end of the Han dynasty, with the Dunhuang manuscripts (discovered in 1900) indicating that the military establishment in the northwest was maintained for most of the Eastern Han period . </P> <P> Following the end of the Han dynasty in 220, China disintegrated into warlord states, which in 280 were briefly reunited under the Western Jin dynasty (265--316). There are ambiguous accounts of the Jin rebuilding the Qin wall, but these walls apparently offered no resistance during the Wu Hu uprising, when the nomadic tribes of the steppe evicted the Chinese court from northern China . What followed was a succession of short - lived states in northern China known as the Sixteen Kingdoms, until they were all consolidated by the Xianbei - led Northern Wei dynasty (386--535). </P> <P> As Northern Wei became more economically dependent on agriculture, the Xianbei emperors made a conscious decision to adopt Chinese customs, including passive methods of frontier defence . In 423, a defence line over 2,000 li (1,080 kilometres (670 mi)) long was built to resist the Rouran; its path roughly followed the old Zhao wall from Chicheng County in Hebei Province to Wuyuan County, Inner Mongolia . In 446, 100,000 men were put to work building an inner wall from Yanqing, passing south of the Wei capital Pingcheng, and ending up near Pingguan on the eastern bank of the Yellow River . The two walls formed the basis of the double - layered Xuanfu--Datong wall system that protected Beijing a thousand years later during the Ming dynasty . </P>

When did the great wall of china fall