Supposedly all it would take would be 1 or more well placed nukes detonated over the US. The resulting EMP could be catostrophic. Fired from a container ship lying off the coast in international waters...
I think the catastrophic EMP theory has pretty much been debunked.
That's very possible, and a very big problem. It would be difficult to prevent. And it would be difficult to figure out just who did it.
For example, was it an Iranian container ship, or was it a Russian one rigged up to look like an Iranian ship? Heck, even the Chinese might try to pull off an attack, then make it look like the Russians were responsible.
These are times when the steady and firm hand of a Ronald Reagan is needed. Instead, we have, well, you know.