Xiongnu
Sinologist Edwin Pulleyblank argued that the Xiongnu were part of a Xirong group called Yiqu, who had lived in Shaanbei and had been influenced by China for centuries, before they were driven out by the Qin dynasty. Qin's campaign against the Xiongnu expanded Qin's territory at the expense of the Xiongnu. In 215 BC, Qin Shi Huang sent General Meng Tian to conquer the Xiongnu and drive them from the Ordos Loop, which he did later that year. After the catastrophic defeat at the hands of Meng Tian, the Xiongnu leader Touman was forced to flee far into the Mongolian Plateau. The Qin empire became a threat to the Xiongnu, which ultimately led to the reorganization of the many tribes into a confederacy.In 209 BC, three years before the founding of Han China, the Xiongnu were brought together in a powerful confederation under a new chanyu, Modu Chanyu. This new political unity transformed them into a more formidable state by enabling formation of larger armies and the ability to exercise better strategic coordination. The Xiongnu adopted many of the Chinese agriculture techniques such as slave labor for heavy labor, wore silk like the Chinese, and lived in Chinese-style homes. The reason for creating the confederation remains unclear. Suggestions include the need for a stronger state to deal with the Qin unification of China that resulted in a loss of the Ordos region at the hands of Meng Tian or the political crisis that overtook the Xiongnu in 215 BC when Qin armies evicted them from their pastures on the Yellow River.
Han–Xiongnu War
In 169 BC, the Han minister Chao Cuo presented to Emperor Wen a memorandum on frontier defense and the importance of agriculture. Chao characterized the Xiongnu as people whose livelihood did not depend on permanent settlement and were always migrating. As such, he wrote, the Xiongnu could observe the Han frontier and attack when there were too few troops stationed in a certain region. He noted that if troops are mobilized in support, then few troops will be insufficient to defeat the Xiongnu, while many troops will arrive too late as the Xiongnu will have retreated by then. He also noted that keeping the Xiongnu mobilized will be at a great expense, while they will just raid another time after dispersing them. To negate these difficulties, Chao Cuo elaborated a proposal, which in essence suggested that military-agricultural settlements with permanent residents should be established to secure the frontier and that surrendered tribes should serve along the frontier against the Xiongnu.