Xiongnu
The Xiongnu () were a confederation of nomadic peoples who, according to ancient Chinese sources, inhabited the eastern Asian Steppe from the 3rd century BC to the late 1st century AD. Chinese sources report that Modu Chanyu, the supreme leader after 209 BC, founded the Xiongnu Empire.
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.