In over forty years of relations with the United States, South Korean decision-makers have had plenty of time to estimate the costs and benefits of acquiring nuclear weapons. The puzzle becomes why South Korea did not develop an operational nuclear capability, given the North Korea threat, the weakening of the U.S. guarantee, a vibrant economy, and an advanced nuclear manufacturing base. This case provides proof that U.S. rewards and threats significantly affect Third World states' nuclear decision-making and that the United States has greater influence with smaller and more vulnerable states than with larger and more technologically advanced states.