No one knows how good an EMP attack would be—would it black out the nation—or just part of it? Would it cause collapse or just make America vindictive. In any case—subs at sea would launch a counter-strike on any nation that attacked us—or where the bomb came from. Any nation would ceast to exist, becoming a place of ruines, death and radiation. No rational people want to die—so an attack might not be coming. Even if Iran gets the bomb—will they hit Israel—and face obliteration?
Actually we’ve got plenty of idea of how an EMP would be. We’ve measured them, we know how they work, we know what kind of electronics are vulnerable. It isn’t some great mystery, we’ve done, and continue to do (under ground with high tech “cheats”), a lot of nuclear tests.
And as you point out, even if it was extremely effective we’ve got plenty of retaliation force it couldn’t possibly effect. That was after all the whole point of mutually assured destruction. Nobody can take us out hard enough that we can’t take them out too.