Still assuming those NATO clowns would insist on playing war, Moscow has already made it very clear Russia would use their awesome arsenal of 5,000-plus tactical nuclear weapons - and whatever else it takes - to defend the nation against a NATO conventional attack. Moreover, a few thousand S-400 and S-500 systems are enough to block a US nuclear attack.
None of this hair-raising Apocalypse Now scenario is even taking into account the Russia-China alliance - the major, game-changing Eurasian story of the 2010s.