The smallest of the three kingdoms was in the South and was readily accessible to the Japanese and was not as xenophobic as the other two kingdoms. The middle kingdom, today's North Korea, was always xenophobic and exceedingly totalitarian.
Nothing has really changed!
Sounds a little like the old three kingdoms of Vietnam. After I read about the 700 history there, I realized that there was little likelihood of the whole thing falling like dominos into China’s pocket.
See Yellow Sea, a SK film from last year that opened around USA two weeks ago. The first segments were filmed in Yanbian.