During the period of Emperor Wu of the Western Han Dynasty, Emperor Wu occupied the northern part of Korea and established Lelang County. Since then, the northern part of Korea has been under the jurisdiction of China until the end of the Western Jin Dynasty, when it fell in the five lakes and sixteen countries. Since then, Yang Di attacked Goguryeo three times, but all failed, and so did Emperor Taizong. It was not until Tang Gaozong destroyed Goguryeo that North Korea returned to China's control. However, the Tang Dynasty made friends with Silla, which had just destroyed Baekje, and Silla surrendered to the Tang Dynasty. Since then, Zhu Chong Pakistan had fought a war with North Korea until the Ming Dynasty perished, but Zhu Chongba failed. After the establishment of the Li Dynasty, the two sides demarcated the Yalu River, and North Korea surrendered to the Ming Dynasty. Since then, it has been followed until the end of the Qing Dynasty. After the Sino-Japanese War, China completely lost its sovereignty over North Korea. Although it was not conquered, Korea was an important vassal state in ancient China, and China always had sovereignty over it. There were few emperors in ancient China. When they split, they were determined to be unified. At the time of reunification, apart from the constant troubles in the northern border of Yuan and Qing Dynasties, the Central Plains Dynasty really had no time to take care of it. Moreover, the ancient emperors of China valued the unification of forms most and looked forward to the grand occasion of coming to North Korea from all directions. North Korea has always been obedient, but it also needs the help of the Central Plains dynasty, such as the Ming Dynasty's anti-Japanese and aid Korea. When the Ming Dynasty perished, North Korea even sent troops to help the Ming Dynasty fight the Qing Dynasty, and China emerged as the eldest protector.