Emperor Wu of the Western Han Dynasty bought Hetao and set up Shuofang County. How did he lose it later?

The war at the end of the Western Han Dynasty led to a large decrease in the population in the north. Shortly after the establishment of the Eastern Han Dynasty, the newly joined Southern Xiongnu moved to the sparsely populated Hetao area. After the defeat of the northern Xiongnu, many Xiongnu tribes went south to join the southern Xiongnu, which led to a great increase in population here. In the later period, the southern Xiongnu was divided into two, and Nan Shan Yu, who was subordinate to the imperial court, led his men to move to Hedong (now Shaanxi). The southern Xiongnu tribe that stayed in Hexi was actually independent, but it was not unified internally. At the same time, a large number of Qiang people, more than Huns, moved inward, and the war with the Eastern Han court continued, which led to a large number of Han people in the northwest moving to Kanto. In this way, the Guanzhong and Hexi corridor areas, which are dominated by farming economy and densely populated by Han people and located on the main roads of east and west, can still be controlled by the central government, while the northern Shaanxi, Ordos and Hetao areas, which are semi-agricultural and semi-pastoral, and are sparsely populated, are completely occupied by various "Hu Qiang" tribes.

In addition, since the end of the Eastern Han Dynasty, the climate has become dry and cold, and the dividing line between farming and nomadism has moved south. Warlords from all walks of life who are fighting for hegemony in the Central Plains have little interest in expanding their territory to the north. The reason why Cao Cao returned to Wuhuan in the north was that Wuhuan befriended Yuan Shao, and after the failure of Yuan Shikai Group, he accepted and supported its remnants, which was equivalent to participating in the civil war in the Central Plains, which Cao Cao could not tolerate. "Hu Qiang" is relatively calm and poses no threat to the Central Plains regime. Therefore, there is no need for Cao Wei to stir up this "hornet's nest" to add chaos, and he has no intention to recover the "useless land" that has long been a pasture and is not suitable for large-scale reclamation.