"Now, I'm not saying we wouldn't get our hair mussed..."

won by whom!?

Mmm yeah imma small brain here and say it cannot be won. There are nukes on subs traveling all over the ocean. Not to mention that the environmental damage and radiation will probably fuck up the world really bad no matter what. Did you know a single nuke in orbit would emp a whole continent and destabilize climate? Great, now you do

My question is "what does this mean for the nature of empires?" — can they no longer truly fall once they enter the nuclear era? How does civil war look?

a single nuke in orbit would emp a whole continent and destabilize climate
That's a bit overblown.
https://en.wikipedia.org/wiki/Starfish_Prime
We've done orbital tests before and while the effects are more than we expected, they are not continent crippling or climate destabilizing.
about 900 miles (1,450 km) away from the detonation point, knocking out about 300 streetlights, setting off numerous burglar alarms, and damaging a telephone company microwave link. The EMP damage to the microwave link shut down telephone calls from Kauai to the other Hawaiian Islands.
It's definitely not good, and there's a reason we all agreed not to do that anymore, but it's really more about the damage it does to satellites that hurts everyone, and the damage being too unstructured to be worth investigating too far.
- That was before we had microchips in everything, and the internet.
  Today, an EMP would have vastly different effects.
The point, as I understand it, is that nuclear war can be technically won by nuking the enemy to smithereens and tanking their nukes as best as possible. The concern here is that because this would technically count as a victory, that it is a battle tactic that elitist assholes in the government have no doubt considered using.
- Maybe you can take out the air fields and the silos before anything has left the ground, maybe, but the subs are already out at sea.
  MIRV makes interception dubious...
- It doesn't matter how well you can tank the enemy nukes when yours alone still fuck the climate and ruin the biosphere.
  But yeah, some numpty with their head up their ass probably plans on ruling over the ashes.
- No more than 10 to 20 million killed, tops! Uh, depending on the breaks.
- Reality isn't a video game: you can't just "tank nukes".
  Even a limited nuclear conflict between, say, India and Pakistan would likely lead to a global food security disaster and could kill up to a third of the world's population – see eg this article (open access). That's using less than 3% of the world's total nuclear stockpile
  edit: welp I didn't notice I was in NCD

A mouse would not invent a mousetrap

I guarantee if mice and rats were smart enough they would make things like mousetraps to harm the mice/rats they don't like.

Yup, they’re called Skaven and they’re assholes to each other