The vulnerability of the US electrical grid has been known for YEARS.  Witness the various EMP Commission reports.  We can thank many administrations for not doing anything about it.  The effects of EMPs and how to deliver a massive one with a nuclear bomb have been known for DECADES.

This is the greatest threat that smaller nuclear powers present to us.  You don't need 5000 nuclear warheads to be able to do this.  And you don't have to take the whole grid down.  Imagine the human, social and economic impact if just one coast were shut down with ONE nuke - even for 6 months.  And not just the grid would be blown but virtually all electronics (computers and anything with computers/electronics like cars, trucks, aircraft, phone systems, etc etc).

Of course any country that would do this would be vitrified by our own nukes, IF their neighbors, who may be our friends OR nuclear powers themselves, let us...Which means Iran and North Korea are probably not that vulnerable to retaliation.