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 infrastructu…
As far as I know, military bases don’t have their own nuclear reactors or coal plants.
EDIT: Apparently the DoD requires diesel backup with enough fuel to last… “one to two weeks.” Meh… if I’m an EMP-empowered nation who hates the US, the grid looks tempting… sure, they can counterattack, but if you make investigating difficult and spread enough pandemonium… all you need to do is keep the government haphazardly distracted and confused for two weeks and you win…