It was the kind of uranium that — if highly enriched — would, by some estimates, provide about half the material required for a crude nuclear device and more than enough for a so-called dirty bomb — a nightmare scenario for U.S. authorities. "I would say that the single largest, most urgent threat to Americans today is the threat of nuclear terrorism," said Graham Allison, an expert on nuclear terrorism. Allison is the director of the Belfer Center for Science and International Affairs at Harvard's Kennedy School of Government and a former assistant secretary of defense. This suitcase's journey was...