Between the northwest and southeast of China, the ancient Great Wall is generally consistent with the natural dividing line of agricultural and pastoral areas. As early as 1 12 1 years ago, road flyover Qiu Chuji said when crossing Yehuling, the first pass in Zhangjiakou in the north, "Climb high and look south, overlooking Taihang Mountain. It's sunny and lovely. In the north, the cold sand declined, and the wind of the Central Plains was isolated from then on. " "Liao History Wei Yingzhi" also said: "The south of the Great Wall is rainy and hot, its people plow for food, Sang Ma is clothed, the palace is inhabited, and the city walls are governed; Between the deserts, it's cold and windy. Animal husbandry feeds on fish, fur and clothes, and converts to disciples at any time to make chariots and horses. This time and place is limited to the north and the south. " This geographical environment determines the confrontation between agriculture, animal husbandry and agricultural nationalities and nomadic nationalities in the history of China, and the two economic nationalities both communicate peacefully and wait for each other. Whenever the cold period replaces the warm period, there are always large-scale nomadic people migrating to the warm areas in the south, and the agricultural dynasty in the Central Plains is facing the challenge from the nomadic people in the north. Among them, the "Five Wild Flowers" around 400 AD, the Khitan, the Nuzhen and the Mongols went south one after another around 1200 AD, and the Manchu entered the customs around 1700 AD, which is the most typical example of the military conflict between the farming and pastoral peoples. The war and relationship between the Central Plains Dynasty and the northern minority regimes has always been the key factor that affects and restricts the social and historical development of China.