The major problem I see with the sober analysis that "Russia won't attack NATO" is that the actors might not be rational. The whole philosophy of MAD, which kept the use of nuclear weapons in check, falls to pieces if one of the players stops acting rationally.

That's not to say I buy the ruminations that Putin has gone crazy, but in an era where you can nestle yourself in any truth you please, it's quite possible that Putin isn't using the truth that we think he is. His acts will be rational, just not rational within the framework that we are using.

One thing that might change the equation of NATO not involving itself directly is a massive humanitarian crisis. I personally think it's inevitable that a no-fly zone will be imposed once the Russians decide to use their air force properly and wipe out massive amounts of civilians while razing a major population centre. Guernica is going to be reflected in Kyiv.

Once that no-fly zone is imposed, all bets are off and escalation is inevitable. The real wildcard is what China ends up doing. They have their own expansionist plans and might use the chaos of war in Europe to accelerate them.