Why did Qin Shihuang build the Great Wall?

In 2 15 BC, Meng Tian, a general sent by Qin Shihuang, led 300,000 troops to attack Xiongnu in the north. Meng Tian, a young man, was both civil and military, and won the trust of Qin Shihuang. At this time, the main force of Xiongnu was in the west of Yinshan Mountain and the north of Helan Mountain. One invaded the north of China and scattered in Longxi and Hetao areas. According to the situation at that time, Meng Tian decided to lead the main force from Shang Jun (now Yulin, Shaanxi Province) to the northern part of Hetao, and another northern army (now Qingyang County, Gansu Province) went out of Xiaoguan, entered the southern part of Hetao, and wiped out the Xiongnu in the Hetao area. After the Huns in Hetao area were cleared, the soldiers were divided into two ways. The main force crossed the Yellow River from the northwest of Hetao and captured Wuyuan and Langshan. An army crossed the Yellow River from the southwest of Hetao, captured the highland of Helan Mountain, and joined the main force of enterprising Wolf Mountain. In the summer and autumn of 2 15 BC, Meng Tian invaded Xiongnu according to the established strategy, and the area south of the Yellow River was quickly recovered by Qin Jun. In the spring of 2 14 BC, Qin Jun crossed the Yellow River from Wuyuan and quickly entered the Langshan Mountains. After crossing the river, an army captured the Helan Mountain area. Xiongnu fled to the north under the fierce attack of Qin Jun. At this point, the original border areas of Qin and Zhao were all recovered, and the battle of chasing Xiongnu in northern Qin was successfully ended.

In order to consolidate the great victory of expelling the northern Xiongnu and safeguard the security of the northern border, the Qin Dynasty adopted a series of strategic defensive measures. The first is to establish local political power and establish Jiuyuan County, which is located in Wuyuan County, Inner Mongolia. Secondly, the Great Wall was overhauled, and the Great Wall from Gao Chao along the yinshan mountains to Yunzhongyuan was built, and the Great Wall from Gaoque along Langshan to Guan Yu was newly built. At the same time, Dai Yun, Shanggu, Yuyang and Youbei Pingxian were ordered to organize manpower to repair the Yunzhongyuan Great Wall connecting Zhao Yan. After years of construction, the Great Wall has become a protective barrier stretching more than 10,000 kilometers from Lintao in the west (now Lintao in Gansu) to Liaodong in the east. Qin Shihuang also ordered large-scale immigrants to fill the border and develop the border economy by combining border defense with reclamation, so as to consolidate border defense. At the same time, Meng Tian led the army to defend Shang Jun (now Yulin, Shaanxi Province) for a long time to prevent the Huns from harassing the northern frontier.