The alliance with the United States has allowed South Korea to build a strong democracy with citizens confident Washington would protect them if North Korea ever acted on its dream of unifying the peninsula under its own rule. But that certainty is fading. North Korea’s repeated threats to launch nuclear weapons at its enemies and its tests of missiles designed for pinpoint strikes on U.S. cities have made South Koreans lose faith in America’s vow to defend their country. The fear is that a U.S. president would hesitate to use nuclear weapons to defend South Korea while knowing North Korea could retaliate with a nuclear strike killing millions of Americans.