China in the Han Dynasty extended into north part of Korea. However, the two countries agreed to boarders along the Yalu about a thousand years ago, and it's been stable ever since. Are you trying to tell us Korea will try and ditch that boarder if it's reunited?
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It was Wei, not Han, that invaded and crushed Goguryeo. The notion that Korea/Chinese border has been stable for a 1000 years really requires a highly colored nationalistic view of Chinese history which asserted everyone who established control over any parts now within Chinese borders to have really been Chinese, and ignores lengthy periods between fall of Tang and rise of Ming during which China effectively lost control of much of northern China, including the area north of Yalu, to various steppe nomad cultures culminating Mongol invasion. It was only during Ming that China unmistakeably reasserted control over north bank of Yalu.
Korea nationalism undoubtedly aspire to a larger version of Korea than the peninsula south of Yalu, and sees area in China with high concentration of ethnic Koreans as natural places that needs to rejoin the motherland. However, they will undoubtedly lack the strength to force the issue in the foreseeable future. But nationalistic politians will undoubtedly use these aspiration to advance their careers at the expense of easy relationships with China.
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