In the ancient history of China, the Three Kingdoms period was an era full of war and political struggle. During this period, the three kingdoms of Wei, Shu, and Wu were in opposition to each other and wars were constant. However, despite the frequent domestic conflicts, there were no large-scale foreign invasions during this historical period. This article will explore the reasons behind this phenomenon.

Firstly, the military defense systems of the Three Kingdoms. The governments of the Three Kingdoms period all attached great importance to military defense, especially border defense against foreign nations. Wei established a strong defense line on the northern border, Shu had steep mountains as natural barriers, and Wu relied on the Yangtze River system to construct a defense system. These military defensive measures effectively prevented large-scale foreign invasions.

Secondly, the internal division and weakness of foreign nations. During the Three Kingdoms period, northern nations such as the Huns and Xianbei were in a state of internal division and weakening power. They were unable to form a unified force to resist the Three Kingdoms governments in the Central Plains. Therefore, even if they had the intention to invade, it was difficult to pose a substantial threat.

Thirdly, the restraining effect of the political situation in the Three Kingdoms. The political struggle and state of war among the Three Kingdoms made all parties have to maintain vigilance and restrain each other. This internal tension made the countries more focused on internal power struggles rather than external threats. This internal political balance avoided opportunities for foreign invasion to some extent.

Fourthly, the use of diplomatic means. The rulers of the Three Kingdoms period were very skilled at using diplomatic means to handle relations with foreign nations. Through alliances, rewards, and marriages, they successfully stabilized the borders and reduced foreign disturbances. For example, Zhu Ge Liang established good relations with the various southern nations during his southern expeditions, ensuring the security of Shu's southern border.

In conclusion, the reasons for the absence of large-scale foreign invasions during the Three Kingdoms period are multifaceted. The military defense of the Three Kingdoms governments, the division and weakness of foreign nations, the restraining effect of internal political situations, and the use of diplomatic means together constituted a relatively stable national defense system. These factors interwove and jointly safeguarded the border security of the Three Kingdoms period, so that although there were constant civil wars during this period, it did not suffer large-scale foreign invasions.

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