To add to that, NATO already said they don't want part on this issue and they wish things could be de-escalated. US said a similar thing, pretty much "best of luck Ukraine, hope you can solve it on your own".
I don't think WWIII is going to come out of this, honestly.
Except, that's patently false considering the hundreds of millions of dollars in arms & training the US has been shipping to Ukraine in the past year. [0][1][2]
[0]https://news.antiwar.com/2021/04/07/us-delivers-military-car...
[1]https://www.cnn.com/2021/12/08/politics/us-ukraine-security-...
[2]https://www.cnn.com/europe/live-news/ukraine-russia-putin-ne...
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.
A coup in process in Russia? Some military / oligarchic faction which already has reached the upper echelons of powers in Russia and they are forcing Putin's hand?
What internal restrains could Putin still retain, for how much time?
Is an escalation of hostilities in the cards? Has this already started and the whole Ukraine operations are just a maskirovka? (this last one is commonly mentioned everywhere these days, it seems everyone suspect the russians are not actually that weak and powerless to win a war, and they are just waiting for to deploy the full war machine against the actual targets).
https://en.wikipedia.org/wiki/Russian_military_deception
Kind of tense wait till this ends ..or it actually start to unfold.
If the news about things like the F35 being only half operational are true, NATO might not be the overwhelming force we assume it is either. Australia is having a lot of trouble making them operational: https://theconversation.com/australia-spent-billions-on-jet-...