I would actually argue that the Central Plains (Northern China) is embedded in the Steppes and Korea as well as China took and built from nomadic traditions. In fact, Korea just borders on the plains and is therefore, much more removed from nomadic influence. Only nomad dynasties, like Yuan and Qing, faced a united peninsula and succeeded in conquering all of it. This applies to Tibet, as well.

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Korea, like Tibet to some extent, is filled with mountains- so much so that the Americans didn't supply South Korea with tanks because they thought the rice fields and the mountains would render them useless. Rivers like the Yalu, Cheongcheon and Nakdong serve as defensive lines. And being a peninsula, the sea-fronts help defend against less than seaworthy adversaries. At various points, Korea was the sea-power of the region along with the underrated Okinawans.

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When the Korean kingdoms of Gojoseon, Goguryeo and Balhae fell in the north, nomad clans like the Xiongnu, Mohe, Khitans and Jurchens filled the vacuum and create a buffer zone in the plains on Liaodong and Manchuria from the Han Chinese. They would often figure it's easier to go West into China, bypassing the Great Wall, than it is to weather the harsh winters surrounded by stout mountain fortresses and skilled archers equipped with bows rivaling their own.

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