Many experts believe that it is almost inevitable that terrorists will soon have the ability to detonate a nuclear weapon in the heart of a major American city. How can we stop them? What are the specific threats that we face and how should we respond to them? Do we face a greater danger from nuclear weapons that may have been stolen from the former Soviet Union or from the clandestine efforts of rogue nuclear scientists? And if the threat has increased since 9/11, why hasn't the United States done more to contain it? Peter Robinson speaks with Graham Allison and Scott Sagan.