To do this you need to set off at least two 270 kiloton weapons about 900 miles apart 200 or more miles above the earths surface. The devices would have to be centered on the long dimension of the US.

There are really only two nations that have the capability along with maybe the desire to do this at this time.

However blowing off nuclear devices above the US would still be viewed as a nuclear attack by the US. Although the EMP would fry most non-protected electronics most of the important military defense systems are shielded for just this scenario. Retaliation would be swift and brutal (within the hour).

I think one of the problems with people that create articles such as these is they are not really knowledgable of what the US nuclear defense and offense systems consist of or what their capabilities are. The countries that have the capability to carry out such an attack DO KNOW the capabilities however and know that such an act would be suicide.

It isn't going to happen.