What missile defenses? And for that matter, the bomb that would produce the pulse can be in space...I'm not really 100% convinced that an EMP will be TEOTWAWKI. First, you'd have to be a suicidal country knowing you're about to be vaporized in return. Then your missile(s) would have to have make it thru the anti-missile defenses. Then it would have to detonate on location. Then it would have to perform like everyone seems to assume it would.
That's a plan with a lot of moving parts and a LOT of risk for pretty much no return.
https://www.iflscience.com/60-years-ago-the-us-exploded-a-nuclear-bomb-in-outer-space-64400
Shut down systems that were a lot "harder" EMP-wise, than current solid-state electronics in the US 1400km or nearly 900 miles from the detonation site. Pop off two or three of those over CONUS and it's "Light's Out."
https://www.goodreads.com/en/book/show/6845861