Our allies can send in special troops to take out the Russian leaders, then hope that their replacements are open to diplomatic resolutions. Why retaliate by killing millions of innocent people, who had nothing whatsover to do with the original decision?
Its a scenario that makes no sense. How far through this campaign of destroying every country on earth would they need to get before the planet is contaminated to the point that the Russians are all going to die anyway - just more slowly?
If a significant chunk of Russia's 3,000 nuclear warheads have landed here, I really couldn't care less whether we were able to 'retaliate' or not. Whats the POINT?