en.wikipedia.org
Duh?
"Mutually assured destruction" in its direct meaning, with the current amounts of nukes, given that in the "world war" would participate all countries, is hardly possible. Too many cities in the world, too few nukes. Most of the world won't be "destroyed".
So, more realistic is to speak about "unacceptable damage" in Russian-American war. But what damage is acceptable, and what damage is unacceptable, depends mostly on possible alternatives. If, say, United States face the imminent and realistic threat of "decolonisation" (meaning genocide of all Americans of European, African and Asian heritage) the death of half of Americans is lesser evil than the death of practically all Americans.
If we assume, that both sides act rationally and their motivation is self-preservation, then, of course, the first goal of any side will be nuclear forces of the enemy. "We want to defend our people, that's why, first of all, we attack your military (first of all, nuclear) forces". That's why there is
- Deterrence Type I (AKA "Providing Stability") or The Credible Capability of the Second Strike. We just saying (and building our system this way): "Even if you attack our nuclear forces, in the moment you choose and on the best for you terms, our survived nuclear forces will be sufficient enough to strike back and kill more your people you can accept, even if they are partly evacuated, partly shelptered".
But also, there is the Detterence Type II (AKA "Providing Multistability"), or "The Credible Capability of the First Strike". It's not just about capability to launch missiles first. It's about capability to attack enemy's nuclear forces and degrade then enough, to allow your ABD intercept significant part of them, and then, suffer quite limited and acceptable damage. Of course, the sides are not going to suffer even "acceptable damage" just for lulz.
And there is the Detterence Type III - capability to fight and win a Limited Regional Nuclear War (especially a proxy war) when both sides are sure about their Deterrence Type I, and try to avoid direct nuclear war, but ready to accept a limited defeat in some regional war.
So, talking practically, if Russia strike the USA first, strike smart and strike precise (after proper preparations, like partial evacuation and sheltering of their population) then US retaliation will be, highly likely, quite anemic and hardly will kill more than few millions of Russians. If the Russians attack only US nuclear forces, and avoid attacking American civilians, and then suggest mutually acceptable (but Russia preferred) terms, there is a good possibility that US leadership will accept it and take measures for damage control and won't retaliate at all.