A lot of old Cold War preparatory war game scenarios start with EMPs blanketing the continental US. It only takes a few (like 1) nuclear weapon detonations in thin atmosphere (a few dozen km up) where the primary heat transfer mechanism is radiative (vs. convective, conductive) to cause enough EMP to take out pretty much the entire continental US grid.

The military is fully hardened against EMP. Civilian infrastructure, not so much.

"The EMP robustness of the civilian infrastructure of the United States can be summarized far less equivocally: it is entirely non-existent. Our civilian telephony, electricity, broadband communications and electronics plants are all naked to our nuclear-armed enemies. They were neither designed, nor engineered, nor constructed nor are they operated so as to survive nuclear explosion effects, even at very great distances – for the 'invisible hand' of the marketplace provides no incentives for EMP robustness, nor penalties for failing to so prepare. Large electric power and telephony systems are known to fail under the effects of solar storms, which impose far smaller electromagnetic stresses than are known to arise from high-altitude nuclear explosions of even modest scales. Consequently, even a modest, single-explosion EMP attack on the U.S. might well devastate us as a modern, post-industrial nation."

https://spp.fas.org/starwars/congress/1999_h/99-10-07wood.ht...