Several ten megaton thermo-nuclear airbursts at a thousand feet over Moscow, Leningrad, Stalingrad to start
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Answered by staggered 500 km air & ground 20mt bursts, zigzagging north to south: result 300-200 million dead, the rest dying from the firestorms; this is an old Russian Cold War plan which they have not forgotten in the intervening 50 years.
We have no defense against Russian ICBMS, or even bomb shelters to climb out of after.
I believe it is referred to as “Mutual assured destruction (MAD) which is a doctrine of military strategy and national security policy which posits that a full-scale use of nuclear weapons by an attacker on a nuclear-armed defender with second-strike capabilities would cause the complete annihilation of both the attacker and the defender.” I don’t think either America or Russia is willing to go that far. I hope not.