This is a premise for a timeline that has interested me for a while. I want to make a TL in which a unified Korean state controls Manchuria (at least into the modern era, ideally up to WWII), and the region is recognized as Korean (similarly to how today it is seen as normal that Manchuria is part of China). This means a historical precedent and a significant Korean population.
Korean kingdoms controlled large parts of Manchuria from 37 BCE to 926 AD (almost 1000 years). The Koreans lost this land when the Khitans, and since then never regained it. China didn't (consistently) control much land in Manchuria until the Ming dynasty. There was a short period in the early 1100s when the Korean general Yun Gwan occupied the former Manchurian territories of Goguryeo and built forts there, however he died soon after. Him not dying so soon is one possibility for a POD, but I'm not sure.
There is of course the problem of what happens after the Mongol invasion...
Is this a plausible POD? Any other suggestions? I'm neither Korean nor an expert on Korean history, so I need some help with this. This is my first post here, I know it's long.