Since the United States dropped atomic bombs on Hiroshima and Nagasaki some 60 years ago, the threat of nuclear war has been a dominant and troubling factor in world politics. For more than half the 20th century, the U.S. and the Soviet Union faced each other with vast nuclear arsenals, but the end of the Cold War brought hope that nuclear weapons finally had become irrelevant. Instead, concern has grown as new members have turned to nuclear weapons for their security in an increasingly dangerous world, including feuding Pakistan and India; meanwhile Iran and North Korea threaten to build their own nuclear weapons. Perhaps the biggest emerging danger is not from nation-states at all but from suicidal terrorists carrying nuclear weapons in backpacks. New preventive technologies and tighter regulations provide hope. But many proliferation experts say it's a race against time.

Roland Flamini is a Washington-based correspondent who writes a foreign-affairs column for CQ Weekly. Fluent in six languages, he was Time magazine's bureau chief in Rome, Bonn, Beirut, Jerusalem and the European Common Market and later served as international editor at United Press International.