Nomadic empire
The Xiongnu were a confederation of nomadic tribes from Central Asia with a ruling class of unknown origin and other subjugated tribes. They lived on the Mongolian Plateau between the 3rd century BC and the 460s AD, their territories including modern day Mongolia, southern Siberia, western Manchuria, and the modern Chinese provinces of Inner Mongolia, Gansu, and Xinjiang. The Xiongnu was the first unified empire of nomadic peoples. Relations between early Chinese dynasties and the Xiongnu were complicated and included military conflict, exchanges of tribute and trade, and marriage treaties. When Emperor Qin Shihuang drove them away from the south of the Yellow River, he built the famous Great Wall to prevent the Xiongnu from coming back.
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.