On 25 June 1950, Communist North Korean troops poured across the border into South Korea, intent on reunifying the country through force of arms. What began as an escalation in Korea's bitter civil war soon exploded into a major international crisis, as first the United States and then Red China intervened by sending hundreds of thousands of their own ground troops into battle to prevent the defeat of their respective Korean allies. The war's first year brought seesawing fortunes on the battlefield. After the Communists captured more than 90 percent of the Korean Peninsula, pushing the South Koreans and their American allies to the brink of defeat, a brilliant counterattack engineered by General Douglas MacArthur quickly drove the North Koreans back across the border. Now the Americans surged forward, driving north toward China in hopes of liberating North Korea entirely from Communist rule. But just as MacArthur declared victory to be at hand, hundreds of thousands of Chinese soldiers unexpectedly poured into Korea, catching the Americans off-guard and sending them into a desperate retreat of their own. Eventually the Americans were able to re-establish a defensive line, ironically located almost exactly at the 38th parallel—the line that had divided North and South Korea before the war began. There, by early 1951, the fighting settled into an uneasy stalemate—a stalemate that continues to this very day, as the Korean War never officially ended.
Korea, the first place where the Cold War turned hot, is thus also perhaps the last place on earth where the Cold War still wages on. Communist and anticommunist troops still stare each other down across the barren no-man's-land of Korea's fortified border.
It is in the Korean War's inconclusive end that we can, perhaps, see its greatest historical significance. In Korea, for the first time, Americans fought a war knowing that their enemies possessed nuclear weapons. This meant that the traditional American strategy of total war—doing anything and everything necessary to achieve total victory—might no longer be possible, as overly aggressive acts might cause the conflict to escalate into an apocalyptic nuclear exchange. In Korea, American leaders had to develop, on the fly, new doctrines of limited warfare, carefully balancing the pursuit of strategic interests against the mortal threat of World War III. They were only partly successful; American intervention saved South Korea from collapse, but efforts to go further to roll back the Communist North Korean regime ended in defeat and bitter acrimony among top American leaders; the reputations of both General MacArthur and President Harry Truman suffered as the Korean situation bogged down into frustrating stalemate.
In Korea, the mighty American fighting force that had conquered Nazi Germany and Imperial Japan just five years earlier proved unable to defeat a seemingly inferior foe. The war in Korea, which cost billions of dollars and more than 36,000 American lives, taught a hard lesson: in the context of a nuclear Cold War, American power had its limits.
And as the subsequent, even more disastrous experience in Vietnam demonstrated, it was a lesson that Americans had a hard time learning.