First, the world is unified, but it is still turbulent. Although the Qin Dynasty was unified, the situation in the Central Plains remained turbulent. The wiped out princes and nobles are still stirring in the local area. Although it seems to have surrendered, it is surging. Coupled with the different cultures of different countries at that time, the people's hearts were not yet stable, and the Xiongnu in the north were spying on the Central Plains. This situation is really inconvenient. Therefore, Qin Shihuang has to face not only the threat from the south, but also the snooping from the Xiongnu in the north, which is really a powerful obstacle. Moreover, the strength at that time was really not enough to support the two battlefields, especially the Xiongnu in the north, which was particularly difficult to fight and the logistics supply could not keep up. Therefore, Qin Shihuang chose the formal strategy of south first and then north. After all, the Han dynasty was so powerful that it was impossible to beat the Huns out even if the national treasury was exhausted, let alone the Qin dynasty, which was just established.
Second, Huns are hard bones. Huns are especially cunning. They want to fight very fast, and they run fast if they don't fight. Guerrilla warfare is also a very strong test for the infantry in the Central Plains. The desert is so big that it really consumes materials and armaments. Coupled with the traffic restrictions at that time, it was too expensive. After all, the Huns are all soldiers, mainly cavalry, with strong mobility, and Qin Jun can't catch up at all. It is necessary to build the Great Wall, which can give full play to the guarding ability of infantry in the Central Plains, so that Xiongnu fighters cannot attack the Central Plains again.
Third, the land outside the Great Wall is a bit difficult to deal with. In addition, the trend of the Great Wall also conforms to the curve distribution of precipitation. After all, in ancient times, people lived on the sky, and there was so little precipitation outside the Great Wall that it was not suitable for farmers to farm. So that place was not so important to the Qin Dynasty at that time. Moreover, the guards around the Great Wall are really too long, which requires a lot of troops and consumes the ability of the army. Therefore, it was the best choice for the situation at that time to build the Great Wall and guard its own gates.