Larry's Excellent Elaboration On The Fate of MAD...

 ... Larry correctly makes the point about growing technological disparity between Russia and the US:

If you are a Russian military planner, you realize that M.A.D is no longer a reality. In the event that Russia believed it faced a genuine existential threat from the West from a nuclear strike, even a tactical hit, the Russian military could present President Putin with a viable plan that would destroy the U.S. nuclear response with limited (albeit horrific) damage to Russia. I am not suggesting that Russia would walk away unscathed. But Russia, with a field tested, integrated anti-ballistic missile defense system, would have a better than even chance of surviving a nuclear exchange with the United States.I will repeat–this is a last resort action and I am steadfastly opposed to it. But I think it should be part of the public discussion. Too many ignorant pundits and politicians in the West disparage Russia’s military capabilities as second rate at best.

Remarkably, and I posted Larry's thoughts on this too, this applies to the United States in so far as the realization of a very real technological gap growing, both in missile and anti-missile systems, which creates a rationale for the US to use a first strike as a preventive measure. It doesn't mean that the probability of these scenarios are high, but they are not trivial either. I agree with Larry, this MUST be the part of public discussion. And yes, I agree, Mutually Assured Destruction is pretty much a dying concept against new Russian technologies being rolled out and deployed in industrial quantities. Do not forget how S-500 is described (presented):

С-500 предназначена для поражения всех имеющихся и перспективных средств воздушно-космического нападения вероятного противника во всем диапазоне высот и скоростей.

Translation: The S-500 is designed to destroy all existing and prospective means of aerospace attack of a potential enemy in the entire range of altitudes and speeds.

So, make your own conclusions. And then there are A-235, S-300V4 and who knows what else is coming, and it is coming.