When was the Hetao area merged into the Central Plains Dynasty?

Before the Zhou Dynasty, the Huns belonging to the Miao people lived in Hetao area (the ownership was disputed). During the Spring and Autumn Period, King Wuling of Zhao extended his territory to Yinshan Mountain and established Yunzhong County, located in the east of Tumochuan Plain. After the Qin Dynasty unified the Central Plains, Meng Tian sent 100,000 troops to drive the Xiongnu out of Hetao, where he settled 30,000 households to defend the border and set up Yunzhong and Jiuyuan counties. At the end of Qin Dynasty, all the immigrants returned to the Central Plains. The Western Han Dynasty 127 years ago, Emperor Wu of the Han Dynasty sent Wei Qing out of the cloud to defeat Loufan and Aries, two kings of Xiongnu, and occupied Hejian, that is, Hetao. Yan Shangshu, the minister's father, suggested building a city in Hetao, plowing fields and raising horses as a base for defending and attacking Xiongnu. Emperor Wu accepted this suggestion and established Shuofang County (now Dengkou County, Bayannaoer City, Inner Mongolia) and Wuyuan County (now Baotou West). 125 Established Xihe County (now northwest of Fugu, Shaanxi Province). The Hetao area also includes Yunzhong County and Dingxiang County, as well as the northern part of Beidi and Shangxian County. At that time, people diverted the Yellow River for irrigation, and the local agriculture developed rapidly and the economy was prosperous. When East Wang Mang was in a state of national tension, Xiongnu reoccupied Hetao, "so the border people flowed into Neixian County". (1) In the early years of the Eastern Han Dynasty, Liu Xiu implemented a austerity policy, and many counties in the west and north were abandoned. In the 26th year of Jianwu (50 years), the Huns clashed and Nandan Khan joined the army. The Eastern Han Dynasty resumed the management of Hetao. "So people from Yunzhong, Wuyuan, Shuofang, Beidi, Dingxiang, Yanmen, Shanggu and Tiba all belong to the mainland. Those sent will be punished (relaxed) to make up for the battlements. Those who send the border people to China will be returned to the counties, and they will be given money for grain adjustment. " (2) Ming Di Yongping five years (62 years), "sent border people, including the county, give silver twenty thousand". Hetao is the focus of these two repatriation operations. During the Han Dynasty, the Huns who joined the Han Dynasty were placed in Hetao. Later, during the Wei, Jin, Southern and Northern Dynasties, Hetao area was an important stronghold of many northern regimes. In the Tang dynasty, it was taken back. In Tang poetry, there is a poem "Orchard under Helan Mountain is completed, and Jiangnan in the north of Saibei is old and famous". During the Northern Song Dynasty, Hetao was occupied by Xixia and Liao Dynasty respectively. During the Yuan, Ming and Qing Dynasties, Hetao was dominated by animal husbandry. In the late Qing Dynasty, some people from Shaanxi and Shanxi moved to the Great Wall, and the Hetao area began to become the most important agricultural area in the northwest. During the War of Resistance against Japanese Aggression period, Fu Zeng, a general of the national army, was stationed here to build water conservancy projects. After the founding of People's Republic of China (PRC), the agriculture here has been further developed.