I think that whoever does it will die in a matter of hours if not minutes. As far as I know all the western nuclear-weapon powers planned an almost immediate response. The only question is how many missiles will be sent on Moscow and will it be coordinated. This eventuality makes me wonder if there is a predetermined hierarchy, if it's the US, France or the UK that are supposed to answer first, if it's coming from a bomber, ICBMS or SLBMS. My guess is that the three countries have subs ready in the Baltics or Black Sea.
@Raoul do you have an idea about the protocol?