Koguryo was a large empire that stretched from present day China to Korea. Part of Manchuria was Korean land, but China took it from them. At the time, they did not care because land was not an issue for them. I would not be surprised if Korguyo had Caucasians, most likely Ainus, Polynesians, Hakkas, and Tocharians, living there, besides Mongoloids. Some people think Japanese language is from Korguyeo Kingdom. Once that Korguyeo kingdom fell, I would suspect many of them left for other parts of Asia, mainly Japan, to Europe, and even America and assimilated with some Indian tribes there.
some of talk here about Koguryo are complete B$.
Koguryo was refered for the first time as a self-dominant barbarian tribe in the Manchuria today under ruled of HAN empire (202BC-220AD).Koguryo had a obigation of give tribute to chinese emperor.
at that time,chinese Han empire set four provinces in south Manchuria and north Korea today,totally called "Yuelan FOUR provinces". Koguryo was derietly ruled by hKoguryo county,Yuelan Jun province"Bthat is the origin of Koguryo's name.
after the collapse of Han empire(220AD),the peace in east asia ended,and lots of barbarian tribes assaulted china,just as barbarian did on Rome empire in the west.chinese had to retreat to the south of Yangtse just as Eeat Rome empire did and the north china falled into the grasp of barbarian, just as West Rome empire did.
Koguryo was one of the barbarians rising up after collapse of Han empire, Koguryo took the chance to get the indenpendence and expanded gradually. By the 5 th century, Koguryo had conquered all "Yuelan FOUR provincesh set up by Chinese in Han empire and ruled south Manchuria and north Korea today. at that time Koguryo was most powerful kingdom manchria and korea.
But after the 6th century, the whole China was united under Tang empire. Tang empire was the actual revival of Han empire and was the most powerful and prosperous empire in the wolrd at that time. And Koguryo was crushed by Tang empire and its allies in Korea peninsula, the Kingdom of Xinluo .
After koguryo was conquered, its territory and people was carveed up by Tang empire and Kingdom of Xinluo . south Manchuria and half people belonged to Tang empire and north korea peninsula and half people was presented to the kingdom of Xinluo as the gift of the emperor. and the kingdom of Xinluo gradually developed to modern Korea.