Wrong question.
What would happen if Russia were to detonate a 1 megaton nuclear warhead in Schwedt, Karlsruhe, and Neustadt an der Donau in Germany.
Or rather, what could possibly be done to prevent the end of Europe.
Gyros are spooled, missiles fuelled and elevated. Sufficient munitions exist to saturate terminal intercept.
The idea there can be a war is blatant stupidity. Russia can literally knock Europe back to the medieval period in 3 hours and Europe lacks the capacity to saturate Russia's terminal intercept in kind.
The notion that nuclear war is MAD is outdated and assumes the U.S. would guarantee a full strategic exchange with Russia on account of Europe. Note, by the way, that the U.S. does not have the ABM density necessary to secure critical infrastructure from such strikes.
Russia can use nukes, not on Ukraine - but on principle nuclear powers - with no fear of retaliation.