And if the US doesn't jump to your aid? If in 2028 Trump goes "you guys had six years to prepare and pissed it away, why should we bail you out?" what can Europe do? They could certainly try putting together a multinational European army and taking it into combat but I doubt there is much political will for that in France or Britain, who crucially possess a lot of the air and seapower.
Even without NATO, the EU countries already have a defensive pact.
Which as a sidenote is why the dismantlement of EU looks like appealing proposition to both Russia, and the US (for different reasons).
The US would make such a war easier, and fewer lives would be lost, given it's tech, and intelligence network. With or without the US, Russia would lose such a war.
Only way nukes play into it is if a shithead like Putin says "fuck it", seals himself in the bunker and hits the nukes. But then we are all cooked, whichever country does that, since mutual assured destruction comes into play.
In terms of political and societal effects, really interesting question worth pondering about. How would the other NATO member countries retaliate if the US wouldn't join in defense. That would be a big betrayal, so I hope that at the very least all US assets are seized, and US companies nationalized across the EU.
