I don’t think the US would provide a counterpart for MAD with Russia right now, but the UK or France could. Obviously the other nuclear powers might get involved as well, but a nuke in Ukraine is going to directly impact the UK and France, whereas it might only have an indirect effect on the others.

I think the French government is more likely to, but I don’t know how their population would react to that. I live in but am not from the west of Germany, and people here have a surprisingly long memory about Russia (they also have a long memory about their own history and are grateful to Russia for ending Nazism, it’s just tempered by the several hundred years of history before the twentieth century and by the perceived brutality of the Soviet military, possibly due to propaganda,). Most Germans I talk to generally don’t want to go to war, but if they were going to, they’d want to fight Russia.

My guess would be that France would likely be similar, but I don’t know.