A Stark Nuclear Warning
What concerns me the most about nuclear weapons is that the technical barriers to building them are getting lower all the time.  They're currently within reach of despotic states of below average scientific capacity.  All that's required is the will to defy the world (e.g. North Korea).  Given how North Korea and Iraq have been treated lately, what despot wouldn't pursue nuclear weapons if he thought he could succeed in building them fast enough to avoid being invaded?In the future, the barrier will be even lower.  Nuclear weapons will likely fall within reach of motivated and capable terrorist movements such as ISIL, although I'd be more worried about organizations that haven't carved out territory that's easy to attack.  Once that happens, things could get very ugly.  There is no such thing as MAD with a distributed terrorist organization.Missile defense systems offer no real answers.  They don't work against state of the art missiles (such as Russia has), nor do they work against smuggled bombs or bombs built within the borders of their target nation.  The kind of surveillance that would be required to adequately secure a nation against such weapons is the stuff Orwell never even dreamed of. It would require utterly pervasive, invasive, and constant surveillance.