The Han–Xiongnu wars or Sino–Xiongnu wars, were a series of military conflicts fought from 133 BC to 89 AD between the agrarian Chinese Han dynasty and the nomadic pastoralist Xiongnu confederacy, although extended conflicts can be traced back as early as 200 BC and as late as 188 AD. The Chinese civilization initially clashed with the Inner Asian steppe nomad tribes (then collectively known as Di) that would later become the Xiongnu during the Warring States period, and various northern states built elongated fortifications (which later became the Great Wall of China) to defend against raids down from the Mongolian Plateau. The unified Qin dynasty, who conquered all other states under Qin Shi Huang, dispatched Meng Tian in 215 BC in a successful campaign to expel the Xiongnu from the Ordos region (which was often used as a staging area to threaten Qin's Guanzhong heartland). However, the subsequent civil war following the Qin dynasty's collapse gave the Xiongnu confederacy, which were then unified into a large nomadic empire under Modu Chanyu, the opportunity to reinvade the Ordos region. After the Han dynasty was established in 202 BC, Emperor Gaozu tried to fight off Xiongnu invasions but had himself trapped in an ambush during the Battle of Baideng, and a truce was negotiated by bribing Modu's wife, thus ending the first Han–Xiongnu conflict. Decades of de jure peace then followed with the Chinese implementing marriage alliances to appease the Xiongnu, who still routinely raided Chinese borderlands. However, starting from the reign of the seventh Han sovereign, Emperor Wu, the foreign policy of the Han dynasty began to change from being relatively passive to using a proactive strategy seeking to permanently remove the northern threat. The tension fully escalated in 133 BC when the Han army unsuccessfully tried to ambush Xiongnu raiders at Mayi, and retaliatory border raids intensified. Emperor Wu then started deploying younger generations of offensive military commanders such as Wei Qing and Huo Qubing, and launched several victorious expeditions to control the Ordos Plateau, Hexi Corridor and Western Regions, eventually pushing the Xiongnu north beyond the Gobi Desert with a decisive campaign in 119 BC. After the death of Emperor Wu in 87 BC, the conflict de-escalated to mostly small border conflicts, although Emperor Xuan and Emperor Yuan each sanctioned major offensives against the Xiongnu during their reigns. The overall strategic Han successes against the Xiongnu allowed the Chinese to project their influence deep into Central Asia, which eventually led to the establishment of a regional protectorate in 60 BC. For the Xiongnu, the situation deteriorated with each setback, leading to erosion of the chanyu's prestige among the steppe tribes, and the subsequent internal power struggles further weakened the nomadic confederacy, fracturing it into various self-ruling factions. The Han dynasty then adhered to a divide and conquer strategy, continuing to use marriage alliances (such as that of Wang Zhaojun to Huhanye) to recruit some against others. During the interregnum Xin dynasty, Wuzhuliu Chanyu waged war in 11 AD after Wang Mang, a consort kin who usurped the Western Han throne, attempted to split the Xiongnu by installing 15 new chanyu. Wang Mang mobilized 300,000 troops against Xiongnu and forced Goguryeo, Wuhuan and various Western Regions city-states to send conscripts and provisions, which led to the mass defection of these vassal states. After the Eastern Han dynasty was established in 25 AD, the Chinese initially found their hands full after the chaotic civil war and could not afford any full-scale mobilizations against the Xiongnu raids; they therefore resorted to continue lobbying amongst Xiongnu faction rulers instead. This went on for another two decades until 46 AD, when repeated natural disasters severely weakened the Xiongnu and forced them to flee north from an attack by Wuhuan. The Xiongnu then permanently split into two groups in 48 AD, becoming the Northern and Southern Xiongnu. The Southern Xiongnu submitted as vassals to the Han dynasty, and served as auxiliaries in Han wars against the Northern Xiongnu and other hostile Donghu states like the Xianbei. The Northern Xiongnu continued to resist the Han and were eventually evicted westwards by further Han expeditions. In 89 AD, Dou Xian led 50,000 cavalry on an expedition that decisively defeated the Northern Xiongnu's main force, causing it to split further. In 91 AD, Northern Chanyu was defeated in the Battle of the Altai Mountains, and they fled west into Dzungaria, where they continued causing sporadic troubles until 151 AD when a 4,000-strong Han militia was enough to defeat them, causing them to flee further west into Central Asia where they disappeared from historical records. The Southern Xiongnu continued to be vassals of the Eastern Han, though they occasionally launched revolts until as late as the Yellow Turban Rebellion era. Nonetheless, by 89 AD onwards, the Han dynasty's main concern had already switched to the Qiang people, who had become a bigger threat than the Xiongnu.