A long time ago. It was during when Korea was three kingdoms, Gorguyeo, Baeckhe, and Shilla. Gorguyeo stretched into China, which is Manchuria. When China took it over, they did not care at the time. It is one of my basis to think that there were other people in the three kindgoms.
well, it seems that you have some confusion on the history of Manchuria.
1,When ALexander conquered Persian Empire on 4th century BC, one of Chinese Kingdoms,Yan Kingdom,occupied the SOuthern Manchuria where Liaonin Province is. At that time, Gorguyeo didn’t got born at all.
2,Gorguyeo is not equal to Korea,just as Carthage is not equal to Tunisia today.
after Gorguyeo got destroyed,most of its inhabitants were relocated to China with only a few flee Shilla.
3, On 221BC, Another Chinese Kingdom ,Qin Kingdom annexed Yan Kingdom , and set up the first CHinese empire.So,After 221BC, Souther Manchuria also became a port of united CHinese empire.
3,After Chinese Yan Kingdom got annexed , some of its nobles had to flee North Korea today and set up a kingdom called “Wei’s Chosun”,because its kings’s family name was Wei.
The ruling class of Wei’s Chosun were almost all Chinese ,but its ruled class mainly were aboriginals.
In about 120BC, Wei’s Chosun got annexed by CHinese empire. CHinese empire set up 4 counties in North Korea today.
4,Gorguyeo you refered was still a small tribe when Jusus got born.
Gorguyeo kept subject to CHinese empire until Chinese empire collapsed in 4th century...as we Know, what Hun detroyed was not only Roman empire,but also Antique CHinese empire..
After Chinese empire collapsed, Korguyeo took the chance to conquer the Chinese 4 counties in North Korea and grew to a powerful kingdom streching from Manchuria to North Korea.
4,In antique sinicized world, it is unreasonable to analyse interstate relation with western mind.
There were no a overlord accepted commonly by people in antique western world.
But in antique sinicized world, Chinese empiror wer commonly accepted as overlord and top judge.