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What's the latest POD possible needed to create an Ainu-dominated Japan by 1900, that's a major world player by the mid 20th century, and creates the fewest butterflies possible for the western world, for as long as possible.
Best case scenario - Ainu dominated Japanese home islands
Medium - Independant or western-controlled (or Chinese) Ainu Hokkaido.
Unacceptable - OTL
Sorta inspired by this thread.