For many years after a 1953 armistice ended the fighting in the Korean War, the scenario most feared by American and South Korean military planners was a reoccurrence of what had started the conflict—a massive invasion of South Korea by North Korea using conventional military forces. To defend and deter against this threat, the United States stationed thousands of American troops in South Korea and pledged to come to that country’s aid in the event of attack. More than a million and a half soldiers remain on watch on both sides of the Korean Demilitarized Zone (DMZ) that separates the two countries, ready to fight at a moment’s notice. Bill Clinton, on a visit to the DMZ when he was president, declared the region “one of the scariest places in the world.”

In recent years...