In the history of Yang Di's three expeditions to Korea, it was actually Koguryo. Koguryo has been established for more than 700 years, and Koguryo has always been a small country on the Korean peninsula. On the Korean peninsula, Baekje and Silla once coexisted with Koguryo, but during the Sui and Tang Dynasties, Koguryo's territory was bordered by Liaodong, and they often harassed the border. At that time, China was also at war all the year round, so they annexed the land in Liaodong.
It was not until Li Shimin became the emperor of the Tang Dynasty that he launched the war to conquer Koguryo. Unfortunately, he died before he defeated Goguryeo. However, Koguryo was eventually destroyed during the Tang Gaozong period, and Silla was the only country on the Korean peninsula.
Later, more than 250 years after Goguryeo's death, a Korean country ruled by Wang appeared on the Korean peninsula. This Korea not only unified the Korean peninsula, but also ruled it for more than 500 years. Later, Minister Koryo abolished the king and became king on his own, which was the later Koryo dynasty. Therefore, in history, Koguryo has little contact with North Korea. Whether it is the royal family or the people, Koguryo has always been a small country on the Korean peninsula, but North Korea has unified the Korean peninsula.