The division of the Korean peninsula lies almost seven decades in the past. The circumstances which drew America into that region’s affairs are long over. The Cold War ended more than two decades ago; the struggle between the two Koreas is no longer tied to a global struggle with a dangerous hegemonic adversary. War on the peninsula would be a humanitarian tragedy, not a strategic disaster.
Washington’s ally has more than recovered from the Korean War. The ROK has sped past the North on most measures of national power. Indeed, South Korea has some 40 times the GDP and twice the population of the North. Thus, the South is capable of defending itself.
Nor do American forces on the Korean peninsula perform any larger role, such as helping to contain the People’s Republic of China. Seoul doesn’t mind being defended against unlikely contingencies involving the PRC — which has no interest in attacking the ROK, a country that would not be easy to swallow, let alone digest. But Seoul would not make a permanent enemy of its neighbor by helping America to protect, say, Taiwan. A U.S. request to use South Korean bases in a war against Beijing for such a purpose likely would lead to a collective nervous breakdown in Seoul.