Any close analogy between “getting out” of Korea and American withdrawal from Vietnam, French withdrawal from Algeria, or British withdrawal from India necessarily fails, because in the sense implied by those cases, the United States has not gotten out of Korea.

As recently as 2004, the United States still deployed 37,500 United States troops in Korea. That year the United States and the Republic of Korea (ROK) agreed to reduce the American deployment to 25,000 by 2008, so we are still in Korea, and likely to remain for some time. The reductions to date have never been intended to culminate in the withdrawal of all troops by a date certain, and their pace was for many years uneven. At the height of their wartime strength, American forces numbered 326,363, in the year following the armistice 225,590, and in 1955 the United States maintained a garrison of 75,328. After that, the numbers seesawed, in part according to the level of perceived threat, so that while in 1956 there were 46,024 American troops in the ROK, by 1964 U.S. troop strength had increased by almost a third to 62,596, and five years later it increased again, to 66,531.

While there are some suggestive benchmarks—for example, the 20,000 United States troops withdrawn in 1971, under the Nixon Doctrine, or the adoption by Congress of the Nunn-Warner Amendment to the 1989 Defense Appropriation Bill, which mandated a reduction in U.S. troop strength in Korea from 43,000 to 36,000 by the end of 1991—one cannot easily select a moment when the United States decided that it would never again have to fight in Korea. Leaving aside provocations—the occasional murder, kidnapping, and torture of American soldiers and sailors, any of which might have triggered renewed fighting—there were for many decades concerns that an inadequate American garrison would expose South Korea to very rapid defeat by a North Korean (DPRK) military both more numerous and better equipped than the army the ROK then possessed. The United States would have again been compelled to make an ugly choice between acquiescence in a conquest, first use of nuclear weapons, or an immensely difficult re-invasion of the peninsula.

Those were the choices that the United States came very close to facing in both 1950 and 1951, first when the DPRK almost effortlessly shattered the ROK’s army and drove a U.S. intervention force into the Pusan perimeter, then again when the Chinese People’s Liberation Army crossed the Yalu, advanced over a hundred miles, forced the naval evacuation of the survivors of the U.S. X Corps, and appeared poised to drive the remainder of the U.S. Eighth Army into the sea.

Only recently has the deterrent power of the ROK’s military come to seem fairly persuasive, and even now the United States will still command all forces in South Korea in wartime, although the ROK may take over that responsibility this year. This means that any analogy to American withdrawal from t...