It was an unprecedented attack years in the making, with some last-minute misdirection meant to give the operation a powerful element of surprise. U.S. pilots dropped 30,000-pound bombs early Sunday on two key underground uranium enrichment plants in Iran, delivering what American military leaders believe is a knockout blow to a nuclear program that Israel views as an existential threat and has been pummeling for more than a week. American sailors bolstered the surprise mission by firing dozens of cruise missiles from a submarine toward at least one other site.