Why did Sanhan become another name for Koguryo in Tang Gaozong?

In the eighth year of Yang Di the Great (6 12), Tian Di invaded Korea eastward. He was ordered to write an inscription saying, "You came to Su Xing to complain, and the emperor made an expedition from the east, so you recovered your merits. That's Imperial Xuan Camp. Six divisions thinned down, three Han dynasties were destroyed, and castration was punished, which was clear and clear. ……"

The author of "Sui Shu" once said: "At the end of the emperor, it was left to Liao, and the weather was not beautiful, so the teacher failed. The second generation is based on the foundation, aiming at the universe, diligently practicing the territory of the Three Kingdoms and repeatedly making thousands of crossbows. "

At the beginning, Emperor Taizong wanted to personally levy Korea and planned to send troops. "(Chu) Sui Liang is bent on going to North Korea with Taizong, fearing that he will regret it." The next day, he made a suggestion.

Epitaph of Niu Wenzong: "In nineteen years, Sanhan disobeyed orders, killed the Sixth Division and sued Liaodong." The three Koreas here are relatively clear, including Silla Baekje, that is, Gao Koryo.

The early epitaph of Liu Ziyang, the viceroy of Anton, wrote: "Except for the viceroy of Anton. A little English saves time and a little depth saves trouble. Three Han Huayang, nine kinds of benevolence. "

In the second year of Yonghui, Tang Gaozong wrote to Fu Yu, the king of Baekje, in view of the successive wars of the three countries on the Korean Peninsula: "As for Haidong, the three countries have a long history and their borders are tied. Modernity is coming, so we have a bad feeling. At the beginning of the war, my heart was not calm. So he ordered Sanhan to protect himself, and ordered Sister Jian to find her to vent her anger, which will still be there sooner or later. ..... "The three countries here refer to the three eastern countries.

In 699, Di, assistant minister of Yingtai, wrote to Wu Zetian, demanding the restoration of the original country of North Korea: "The monarch of the three Koreas is the leader. I sincerely hope that your majesty will return to his hometown with the meaning of life and death. " Sanhan here is also Gao Koryo.