Welcoming the leaders of 47 nations to Washington to discuss what he called “an unprecedented threat,” President Obama described in ominous terms the potential for nuclear terrorism. “Just the smallest amount of plutonium — about the size of an apple — could kill and injure hundreds of thousands of innocent people.” The possibility that a terrorist group could get its hands on a nuclear weapon is, the president added, “the single biggest threat to U.S. security, both short term, medium term and long term.”

At the conclusion of the conference, when everyone had agreed to voluntary measures to secure loose nukes, the president pronounced himself very satisfied and confident that the world will make “enormous progress” on controlling nuclear proliferation.