How did Qin Shihuang strengthen his control over the frontier?

After Qin Shihuang unified the six countries, in order to maintain the stable and unified political situation of the country, strengthen the control and contact of the central government over various places, especially the border areas, vigorously build a nationwide land and water transportation network, set up post roads at the same time, and promulgated relevant laws to ensure the smooth flow of post roads. For example, from 220 BC, Qin Shihuang ordered the recruitment of a large number of people, which lasted 10 years, and finally built a traffic trunk line all over the country-"Chidao". "Chidao" is centered on Xianyang, the capital city, extending in all directions and leading directly to the frontier. Historical records record that "Qin Weichi is famous all over the world, poor in the east and Yan Qinan in the south, above the rivers and lakes, and the seaside scenery is complete." The road is fifty paces, and the tree is three feet high. The outside is thick, hidden in the golden vertebra, and the tree is loose. " After the completion of the Equator, Qin Shihuang ordered the construction of a "straight road" with a total length of "1,800 miles" (about 700 kilometers today) from Yunyang in the south to Jiuyuan in the north (now Inner Mongolia occupies the west of Baotou City in the autonomous region). "Straight Road" is an important part of the national land transportation network, and it is a special military transportation line newly built to defend the Huns. According to historical records, the north-south entrance of this national defense traffic artery is generally opposite to the south, intersecting with the Great Wall, another national defense project after Qin reunification, and supporting each other, so it is called the "straight road". At the same time, the road conditions are good, which can be used for the smooth and rapid passage of large troops composed of cars, walking and cycling. There are also military stations for storing military equipment and garrisons on the road, which can quickly support the Great Wall defense line with materials and troops at any time. Once there are policemen in the border areas, the soldiers along the Great Wall will immediately light bonfires, so that the border guards can make corresponding defense arrangements according to the enemy's situation, and the central government can also recruit mainland troops to help them along the straight road, which improves the mobility of the troops. Therefore, after the construction of the "Straight Road", it immediately became an inseparable and important part of the northern border defense system of the Qin Dynasty, which greatly strengthened the connection between Guanzhong, where Xianyang in Qin Dou is located, and the northern Hetao area, making the Huns easily afraid to invade the south. Until the early Han Dynasty, the Han Dynasty lost the strategic position of Hetao. Because of the "straight road", Xiongnu still dared not invade directly from Jiuyuan along the Yellow River basin, nor did they dare to stay in Henan for a long time. Even in the middle of the Tang Dynasty, in order to defend the East Turkistan, which was once revived, the Tang Dynasty also built three surrender cities in the east, middle and west to control the Yinshan defense line. The "straight road" still played a role in connecting the military strongholds in the northern frontier. In addition, in the process of conquering southwest China, Qin Shihuang mobilized a lot of manpower and material resources, and built two land routes in southwest minority areas (starting from Yibin, Sichuan, passing through Zhaotong, Yunnan to Qujing, which is a "new road" connecting Guangdong, Guangxi, Hunan and Jiangxi, and crossing Wuling) and a lingqu (Xing 'an Canal) connecting Lingnan. The construction of "Chi Dao", "Straight Dao", "Wu Equator", "New Road" and Lingqu provided a convenient military transportation network for the Qin Dynasty. In the event of a war, the central government can concentrate the necessary forces in the shortest time, maneuver to the required strategic direction, and respond quickly, which will help eliminate local separatism, safeguard national unity, and effectively defend the frontier and national defense.