Why did the navy of the new China team defend the maritime border of the motherland when there was no coastal defense in old China? Can you give me an example?

First of all, the development of China's navy in the old China period was not perfect. Moreover, foreign aggression and successive years of civil war in the old China period caused unprecedented losses to our people, economy and national defense construction. In the 1970s after the founding of New China, we had a real naval force in China. From then on, our navy began to undertake the mission of defending our country and destroying the sea until the 20 th century. China's naval power construction has gradually caught up with Europe and the United States and other countries. Although it cannot be compared with the above-mentioned countries in some technical aspects, our navy will certainly rely on continuous innovation and development to defend the security of our territorial waters, and will also defeat all enemies who have committed crimes in the past and defend our territorial security.