This is the plot in "One Second After" by Forstchen.If a nuke or nukes are detonated above the surface of the earth strategically, and therefore they create a magnetic pulse that knocks out our various defenses as is based upon modern technologies now, and if they are defenses that are depended upon satlelite signal systems, electricity/the grid, and various other ways we utilize such things, then an invasion would come soon after I would think. Such a strike would probably be used in order to soften up the area of entry if we're used in such a way.
If you have not read the book it is a really good read.
It is one of those books that you cannot put down once you start reading it.