What does "immigration to secure the border" mean in the strategy of strengthening the border?

Historically, nomadic people in the north have irregular movements, are good at riding and shooting, and have strong military advantages. Their attack on the frontier was extremely sudden. As Chao Cuo said, "The livelihood of the Hu people is out of reach and it is easy to disturb the frontier." Therefore, the central plains dynasty was at a disadvantage in defending against the harassment of nomadic people, which can be said to be impossible to prevent. As long as one place is broken, it is thousands of miles away. Therefore, it is difficult to achieve the ideal effect simply by taking military preventive measures. Meanwhile; Because the frontier is far from the mainland, it is neither practical nor practical to station a large number of troops, and once something happens to the frontier. Thousands of miles to help, less is not enough to resist, more is a waste of money, and the country has a heavy burden. Chao Cuo analyzed in his "Supporting the People and Relying on Sparseness" that it is not the way to rely solely on the central government to send troops to defend against the Huns. Constantly recruiting grain, grass and soldiers, corvees and taking turns to guard the border will only make the country overwhelmed, the people restless, the defense is difficult to work, and the border crisis cannot be truly solved. And "moving to the real border", that is, moving to the border before and after military conquest, can not only provide material guarantee for military operations, but also move to idle places on a large scale after repelling the invasion and consolidate the existing achievements of military conquest. Therefore, it is of great strategic significance to move the residents of the Central Plains to live and live in the border areas in a planned way, which will strengthen the border defense and stabilize the border areas.

In terms of economic layout, the Central Plains, represented by the East in history, developed earlier, with dense population and narrow land. Therefore, emigrating from the economically and culturally developed middle and lower reaches of the Yellow River to the border areas not only conforms to the law of economic development, but also promotes the development of the border areas and enhances the comprehensive strength of the country, which is a strategic move with multiple objectives.