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So this is a challenge and a related question-

Could Korea and Japan have launched into large scale expansion without finishing the other one off first? In OTL, Korea never expanded beyond its peninsula politically, and in its OTL historic bids to expand, Japan started in Korea. When they failed in Korea they quit. When they succeeded 300 years later they went after additional targets subsequently.

Could we have plausibly had a situation in the last two-thousand years where both expand simultaneously. This could come in any flavor you'd like. The first that comes to mind is the Atlantic model - the European Atlantic states England, Netherlands, France, Spain, Portugal, checked their ability to take over each other, but managed at the same time to acquire vast overseas territories. The other model is splitting the land and sea domains, wherein Japan expands mostly to the south to additional island or overseas territories, and Korea ends up expanding on the Asian mainland, ruling Manchuria, at least invading China and Russia, and possibly succeeding in holding expanded realm there for several decades.
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