All it takes is two EMP detonations. One over Arizona … the other over Ohio.
Can’t control the spread of the emp effect! A night side emp would be more wide spread, a day side emp would be more limited in spread due to the effects on solar wind on the magnetic field lines. Geo graphic features suct as high mountain ranges, salt water, or mixed plateaus also effect emp spread. Dozens of local high radio/TV and cell towers in given regions will dilute emp effects acting like lightning rods.
Also emp blasts on the scale of the Starship prime sub orbital blasts over America(which is not ocean salt water) might cause much damage but it will also cause damage to Canada, Mexico and much of Cental and South America and damage to Caribeean countries as well but it won’t hurt our subs or any of our fleets at sea. The whole western hemisphere, smarting with damage in many areas will be pissed and wanting payback! The damage won’t be confined just to the continental USA.(Cuba will be affected)
Still much infrastructure will survive, enough to allow an obliterative resonse to any rogue nation that tries it!