Here is why even a first nuclear strike against Russia will not prevent a retaliatory nuclear holocaust. The Russians have developed a “dead man’s switch” defence system known as “Dead Hand,” that will automatically launch every available nuke should Russia face nuclear annihilation, or be destroyed. It is not clear when the system gets activated. And, that is the worry, since there may not be a reason to prevent an all-out launch even in a limited nuclear exchange. This seems infinitely dangerous, and all the more reason not to have war with Russia over the Ukraine. The chattering class interest in that place seems to have gone off the boil.
https://www.rbth.com/science-and-tech/334565-dead-hand-5-questions-about-the-scariest-nuclear-system