(1) The recovery and development of the North-South economy and the narrowing of the gap between the North and the South laid the economic foundation for the reunification of the Sui Dynasty.

As the center of China's social, economic and cultural development-the Yellow River Basin. From the Eastern Han Dynasty to the Northern Wei Dynasty before the reunification of the Yellow River Basin, after many wars, the social economy was seriously damaged and the population was displaced. Although the northern economy was restored and developed through the efforts of Cao Wei and the former Qin Dynasty, the time was short. After the Northern Wei Dynasty unified the Yellow River Basin, Emperor Xiaowen carried out reforms and implemented the equal land system and the three-long system (local grass-roots administrative system). The implementation of the "three systems" of rent adjustment was of positive significance at that time. Farmers have increased their land through reclamation, and rent modulation has reduced their burden. Farmers have certain personal freedom, which has improved their enthusiasm for production and played a positive role in the recovery and development of agricultural production in the Northern Wei Dynasty. In particular, the implementation of the land equalization system has greatly increased the government's income, and "the people are like people, and their customs become customs in years", which has brought unprecedented prosperity.

During the Three Kingdoms period, the southern economy was only in the primary stage of development, and the real development began in the Southern Dynasties. In the Southern Dynasties, (1) "People fled from the Central Plains and crossed the border in Li Nan, and overseas Chinese set up animal husbandry departments in Guangling, Danyang, Cheng Nan and non-old places. The tiger's mouth invaded south, and Huainan people crossed the river. " Almost because of the migration of a large number of people to the south, on the one hand, the labor force in the south has increased (according to statistics, on the other hand, the farmers who moved to the south have brought advanced production technologies and tools. Coupled with the fertile soil and suitable climate in the south, the rulers implemented a series of measures conducive to agricultural development, which greatly improved the social productivity of the south. Although the economic development of the north was higher than that of the south at that time, after this period of development, it can be said that the economy of the south has gradually surpassed that of the north, narrowing the economic gap between the north and the south. Therefore, the economic development of the South and the North needs to break the North-South confrontation in order to promote economic and cultural exchanges.

(2) The great ethnic integration between the North and the South provided ethnic conditions for the reunification of the Sui Dynasty.

Since the end of the Eastern Han Dynasty, due to the increasingly corrupt politics, the unified multi-ethnic empire has fallen apart. After the brief reunification of the Three Kingdoms and the Western Jin Dynasty, the situation of coexistence of sixteen countries in the Eastern Jin Dynasty and confrontation between the Southern and Northern Dynasties reappeared. During the 300 years of regime division and frequent wars, China society was in a huge turbulent vortex. At the same time, due to the great ethnic migration and great ethnic integration, the second unprecedented great ethnic integration appeared in the history of China. All ethnic groups closely related to the Han nationality and its predecessor, Huaxia nationality, suddenly accelerated the process of ethnic integration after appearing on the political stage in the Central Plains, and they gathered together discretely. Many ethnic groups that have established political power have merged with the Han nationality. Two-way or multi-directional migration and convection between the northern and southern ethnic groups are the characteristics of ethnic integration in this period. That is, some Han people went to the surrounding areas, and the surrounding ethnic minorities came to the mainland.

In the north, Xiongnu, Xianbei, Jie, Shi and Qiang, known as the "five lakes" in history, came one after another and established many political powers in the Yellow River basin. Generally speaking, no matter which nationality is in the dominant position, most of these national powers are related to the clan class of the Han nationality. At the same time, the combination of political forces often takes various forms of alliances. Northern nationalities migrated to the Central Plains and established political power. One of the most direct consequences is that Hu people and Han people live together in the Central Plains, which provides a prerequisite for the integration of all ethnic groups. In the long-term coexistence and intermarriage with the Han people, these ethnic groups have been interdependent, absorbed each other and established countless contacts. Gradually, the differences between them and the Han nationality in economy, culture, language, costumes, surnames, customs and even religious beliefs gradually narrowed and gradually merged with the Han nationality. For example, by the late Northern Wei Dynasty, the ethnic minorities such as Xiongnu, Jie, Shi and Qiang had disappeared from history, while Rouran, Tuguhun and Zile gradually merged with the Han nationality. By the time the Sui Dynasty unified the Yellow River Basin, almost all the ethnic minorities who moved from the north to the Central Plains were Han.

At this time, the cultural gap between the North and the South is gradually being eliminated, and cultural exchanges and understanding are constantly being strengthened. The emergence of great ethnic integration has made the people who are deeply troubled and suffering urgently demand peace and stability and realize national reunification.

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