Later, the Tang Dynasty joined hands with Silla, a native of Korea, and eliminated the other two major forces. Koguryo territory returned to the Tang Dynasty. Later, the Tang Dynasty wanted to further rule Korea and then triggered the Luotang War, which ended in failure in the Tang Dynasty, and Silla unified most of North Korea's territory. This period is also called Silla period.
In the Yuan Dynasty, the suzerain-vassal relationship between North Korea and China was formally established, and it continued until the Qing Dynasty. At this time, North Korea publicly recognized the status of the Qing Dynasty in the kingdom of heaven, and then sent envoys to Beijing every year, and used the year number of China. For North Korea, this advantage is that it not only stabilized its own throne, but also got the protection of China when it was invaded by others.
In this way, this relationship was maintained until the Sino-Japanese War, which ended in China's defeat. Therefore, since then, the tribute conferring system centered on China has completely collapsed, and the relationship between China and North Korea has changed. It is no longer the relationship of maintaining the system, but the relationship of lacking lips and cold teeth.
So when the United States invaded North Korea, China would spare no effort to help. Although China was poor at that time, he resolutely sent volunteers to the Korean battlefield, which of course ended in victory.