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The emergence of a prosperous and democratic South Korea has benefited the U.S. and the rest of the world—it’s one of the great post-World War II success stories. Americans have special reason to be satisfied, since Washington’s defense shield enabled the ROK to develop despite North Korea’s threats. But the South no longer needs U.S. support, which by now is only a source of military unpreparedness, the root of the Korean conundrum. Peoples of the two nations should remain friends—cultural, family, and economic ties do not depend on military deployments. And the two governments should cooperate in areas of shared political and military interest. But it is high time for Seoul to shift from security dependent to security adult and solve its strategic...