Imagine you're a terrorist with a single nuclear weapon. You could wipe out the U.S. city of your choice, or you could decide to destroy the infrastructure of the entire U.S. economy and leave millions of Americans to die of starvation or want of medical care.
The latter scenario is the one envisioned by a long-running commission to assess the threat from electromagnetic pulse, or EMP. The subject of its latest, and little discussed, report to Congress is the effect an EMP attack could have on civilian infrastructure. If you're prone to nightmares, don't read it before bedtime.