Suppose the attack could be traced to a freighter laying off the east coast of the US. A freighter programmed to destroy itself after the attack. Who put that freighter there? The Iranians? Or maybe it was the North Koreans, but made out to look like it was the Iranians. Or maybe it was the Russians...the Chinese...
This is what happens when no one respects the US anymore.
If an EMP attack was successfully made, our ability to retaliate would depend on the extent to which our command and control infrastructure was impaired.
All shipping and nukes are monitored 24x7.