According to Zhao He's Biography of Xiongnu in Historical Records, during the Warring States Period, the Nanchangcheng and the Northern Great Wall were both built by Zhao, with completely different functions. The Great Wall of Wan Li began in the Spring and Autumn Period and the Warring States Period and ended in the late Ming Dynasty. At the end of Qin Dynasty and the beginning of Han Dynasty, Xiongnu crossed the Great Wall built by Qin and Meng Tian in the Central Plains War and was bounded by the Great Wall of Qin, Zhao and Yan in the Warring States Period. Due to the disrepair of the Great Wall and the scarcity of defenders in the north, the powerful Xiongnu continued to plunder the Great Wall and went deep into Daigu, Taiyuan, Xihe, Shang Jun and Beidi counties. Emperor Gaozu, Hui Di, Wendi and Jingdi were forced to adopt a pro-Xiongnu policy. Marry a princess to be E Shi (queen) for Khan and give him a lot of wealth. However, even this dilapidated Great Wall has played a role in military defense to a certain extent. If it weren't for the Xiongnu army, it would be impossible to enter the Great Wall and plunder. Secondly, if the defenders are effective, even with the Xiongnu army, they can't enter the Great Wall. Like Li Guang, Cheng Wu is also a famous Great Wall Guardian. He was very strict in running the army and was always on guard against the Huns' surprise attack, which also showed that the Great Wall of the Warring States period played a certain role. Later, Wei Qing, Huo Qubing, GongSunHe and Gong attacked the Huns, all of which were based on Zhao and Qin Changcheng. This shows that until the early Western Han Dynasty, the Great Wall of Qin, Zhao and Yan in the Warring States period was still playing a military defense role, although its defense ability was much worse than that in the Warring States period. It was not until Emperor Wu of the Han Dynasty drove the Huns to Mobei, restored the Qin Changcheng built by Meng Tian and built the Outer Great Wall that the Great Wall of Qin, Zhao and Yan in the Warring States was abandoned and the military defense function ended.
Ancient Great Wall
The Great Wall of Qin, Zhao and Yan during the Warring States Period has important historical value. First of all, it shows that only by building the Great Wall and increasing a certain number of troops can the lightning attack of nomadic cavalry in the north be prevented. Without the Great Wall, even with a large number of infantry and cavalry, it is still impossible to defend. The historical facts of the Warring States period proved this point. Because the Great Wall was an effective military defense project, it was later adopted by Qin Shihuang. After Qin Shihuang expelled Xiongnu to Mobei, he ordered Meng Tian to build a new Great Wall immediately.