Barack Obama grew up long after the 1950s air-raid drills that sent students scurrying under their desks, but the mushroom cloud was never far from his imagination. He wrote his senior thesis on nuclear arms reduction and quoted reggae star Peter Tosh in an essay about the "flowering of the nuclear Freeze movement" for a student magazine. Now that onetime activist possesses the power to summon the world. At the Nuclear Security Summit in Washington, he gathered representatives of 47 nations (including 38 heads of state) for the largest diplomatic event convened by a U.S. President since 1945. Obama's goal--a nuclear-weapon-free world--still sounds quixotic, but real incremental progress is being made. In March he cut a deal with Russia to reduce Cold War stockpiles. At the summit, he secured commitments from Ukraine, Chile and others to safeguard nuclear materials. And in May he hopes to finalize new sanctions to punish Iran for its nuclear ambitions. These are accomplishments of a statesman who dreamed, as a student 27 years ago, of bringing the wisdom of "Thoreau, Jefferson and Whitman to bear on the twisted logic" of the nuclear arms race.