Over the course of a single century, South Korea has undergone one of the most extreme demographic transformations in modern history, making it a landmark case study in demographic whiplash. Its journey began with high growth fueled by mass immigration and a post-war baby boom, followed by one of the world's most aggressive and successful policy-driven fertility declines. This reversal was so profound that it created the unique and complex demographic challenges the nation confronts today, tracing a remarkable arc from an era of turmoil and expansion to the contemporary reality of an aging society with critically low birth rates.