First of all, the historical frontier is a dynamic space or region. In the special historical evolution of the Chinese nation, with the struggle and integration between tribes or races, the rise and fall of various political community forces, the distribution of territories, administrative regions and ethnic groups often changes, and the expansion and contraction of territories and the reform of administrative regions occur at any time, which is not as great as it has been since ancient times, or remains unchanged for thousands of years. One dynasty belongs to the border area of China, and another dynasty may be brought under the administrative jurisdiction of the Central Plains dynasty and become a part of the center, while the area once brought into the Central Plains system may become a border area again in some dynasties. As Wang Fu of the Han Dynasty said: "If Liangzhou is lost, the three assistants will be the side; Sanfu's wife is the fate of Hongnong, and Hongnong's entry is the fate of Luoyang. In the past, although the East China Sea was exhausted, there was still an edge. " For example, in the Tang Dynasty, people regarded the Sanggan River as the "northern frontier" of China, crossing the Sang Yu River to the north was the frontier area, while in the eyes of the Southern Song Dynasty, it was the northern frontier from Hongze Lake to the north of the Huaihe River. Lu You's so-called "poor border refers to the Huaihe River, and foreign countries regard Beijing as a pheasant" is proof. In the prosperous Tang Dynasty, Tianshan Mountain and Congling Mountain were the westernmost points, but in the late Tang Dynasty poet Bai Juyi's era, because Liangzhou (now Wuwei, Gansu Province) was besieged by Tubo, Fengxiang, Shaanxi Province also became the frontier to the west. The so-called "Anxi Wan Li is in Xinjiang at ordinary times, and today the frontier is in Fengxiang". It can be seen that the diachronic fluctuation of the frontier has led to the dynamic change of China's historical territory, and its composition mode shows the characteristics of stages, so the frontier issue plays a particularly prominent role in the development of the Chinese nation. Today's China's frontier is the result of such a complicated historical process to a certain extent, and it is gradually formed and fixed with the formation and development of a unified multi-ethnic country.

Secondly, the "frontier" in the history of China to be studied in this paper is the marginal area around the central area of Chinese civilization within the unified multi-ethnic empire in China's history. The distinction between the central area and the peripheral area mainly lies in politics, geography and culture. As early as the 1940s, Mr. Gu Jiegang pointed out that "the frontier, that is, the outer edge of a country's territory, is geographically different from the mainland, but it is integrated with the mainland in terms of national sovereignty and political system. This meaning covers all countries in the world. " This view is still enlightening today.

The so-called politics, that is, administratively, is the edge of the administrative control of the political system of the Central Plains Dynasty (political unity, especially the Chinese Empire in the era of great unity). The Central Plains Dynasty did not exercise administrative jurisdiction, but was ruled by border ethnic minorities. It should be noted that in the history of China, the relationship between the Central Plains Dynasty and the frontier nationalities, tribes and political power was varied, and the realization of state power often presented two extreme situations: "One was highly centralized rule. Even military control; The other is a high degree of local autonomy. As for which way to implement it in a certain place, it depends on local conditions or time. Therefore, in a sense, China's frontier in history is formally a transitional area from the ruling center of state power to the outside, that is, from governance to ungovernable specific areas. " Thus, it constitutes the pattern of the hinterland of the Central Plains Dynasty and the "Four Foreigners" frontier under the unified system.

As for culture, generally speaking, due to natural, cultural and social conditions, these areas are often ethnic areas with small population and vast territory. Living in a nation other than the main nation. They have their own relatively unique economic life, social forms, lifestyles and cultural types, which are different from the Han areas in the Central Plains in terms of culture and customs.

At the same time, geographical factors are also one of the bases to distinguish the center from the edge. Mr. Gu Jiegang once said that an important feature of China's frontier lies in its geographical particularity: "The foothills of the plain, where boats and cars are clear, are called inland areas, and those who drive camels and float rafts in the desert are called frontiers; Those who are well dressed and stand out from the crowd are called frontiers. " In the writings of China literati. "Beyond the Great Wall", "within the territory" and "outside the customs" are almost synonymous with the frontier.

Third, although the central area and the peripheral area are dynamic, they have their relative stability throughout history. As the saying goes, "African-Americans in the border areas, four generations living under one roof." Until the invasion of western powers, the pattern of "four generations" in China was basically maintained. It is in this sense that the historical frontier is characterized by the combination of stability and volatility. Therefore, when using the concept of "frontier" in this paper, for the convenience of discussion, it generally refers to the marginal areas of China territory in history.

It must be pointed out here that it is not appropriate to regard only the area between the political community established by ethnic minorities in history and the Central Plains dynasty as a historical frontier. As American geographer Irving Latimer thinks, from the perspective of the whole inland of Asia. The Great Wall of China is a center, and there are two social entities in its north and south, namely, grassland society (empire or tribal community such as Xiongnu Empire, Turkic, etc. ) and the agricultural empire of South China. The lasting contact between the two societies along the Great Wall has formed an interactive relationship. "China expelled some' backward tribes' who originally belonged to the same race as the Han ancestors from an environment conducive to the establishment of intensive agriculture in China society, thus contributing to the establishment of grassland society." The region between them then presents a kind of "frontier form", in fact, it only regards the zone between the internal political communities of the Chinese nation, that is, the "inner border" in history, as the frontier of the whole historical territory of China. This view ignores the historical reality of the pluralistic and integrated development of the Chinese nation, and in a certain sense, it still does not break away from the pattern of the dynasty system.

Fourthly, the traditional concept of China's frontier management discussed in this paper refers to the guiding ideology of the Central Plains Dynasty (unified regime, including the unified regime established by ethnic minorities) in the history of frontier management. The general plan of border management (also called the general plan of border management) refers to the overall strategy formulated and implemented by the unified dynasty in order to unify and stabilize the frontier, as well as the principles and methods for planning and guiding these strategies. The reason why the era of great unification is represented is mainly to facilitate the discussion of the internal relationship between the thoughts and strategies of border management in past dynasties and the development and evolution of the frontier, rather than taking the Han nationality or the unified Central Plains regime as the only representative of China because of the central theory of the Central Plains Dynasty. As far as historical reality is concerned, the relationship between the Chinese empire and the frontier is more prominent in the era of great unity, and it is also typical and practical to study it.