A single high-altiltude electromagnetic pulse from a nuclear detonation could do a lot of damage to the US. Launching it off the deck of an oil freighter along one of our coasts would make it hard to intercept and difficult to tell where it came from. The Iranians are just crazy enough to try a stunt like this knowing full well that MAD applies.
Yes and no.
EMP is dangerous, yes.
A single high-altitude electromagnetic pulse could do a lot of damage, yes. (You'd need more than one to completely cripple an area as big as the Lower 48, but even just three or four small New England states from one detonation would certainly qualify as "a lot of damage.")
However, to get the sort of EMP you're talking about, you need a
really big detonation--dozens of megatons. Thirty megatons, say, at an altitude of 50 miles or so.
And megaton-level yields are not easy to get--certainly not with fission weapons, but not even with fusion weapons. The issue is that the chain reaction that creates the power of the bomb needs the reactive mass to stay clumped together through enough generations of reaction to achieve the exponentiation necessary: separate the reactive mass out of a tight clump and the chain reaction instantly falls apart.
So the question is, how do you hold together a reactive mass with a thirty-megaton potential yield for long enough that it can achieve that yield, before letting the explosion begin to expand? And the answer is, mostly you don't.
You certainly don't get it by firing a half-critical mass down a howitzer barrel into another half-critical mass, or by setting off a spherical explosion around a subcritical mass to compress it into a critical mass. From that, you get double- to triple-digit kiloton yields.
So while it's a good thing to think about EMP, and an even better thing to harden electronics against it, it's not something we have to worry much about from a third-world Muslim country.
As for launching a ballistic missile from an oil freighter to almost low-earth-orbit altitudes anywhere near a populated area with skin-paint air-traffic-control radar, I can't believe it'd be all that difficult to identify the launch point.
