Unless a NATO country comes under direct attack or there's casus belli, say a number of NATO jets were shot down, I can't see NATO fighting directly.
Perhaps if there was some form of mass atrocities too. But, the real case is that it appears as if they will not have to as long as the direction continues.
If the news of NATO countries providing jets is too, I suspect that we, the US and other countries will agree to cover their airspace for the foreseeable future.
They'll provide their jets for Ukraine to use, we'll cover them and then over a period of time they'll be provided F-16s or something similar.
I suspect the same will happen with PGMs, which the Ukrainians will use to hit the Russian convoys, logistics depots/camps and command and control.
We'll then offer them weapons to restock their arsenals. Russia is showing itself to be (at best) a second tier army with the moral and desire of a third tier.
As long as we keep the Ukrainians stocked up to fight, I can't see Russia coming out of this well militarily or politically; they'll be back in the doldrums.
I took it with a pinch of salt, but before I was told that Russian casualties (not deaths) are reaching nearly ten thousand, with deaths in the low thousands.
The same person said there's reports of growing 'disharmony' at military camps back in Russia. If the army turn on Putin, it could all unravel rather quickly.
Rightly or wrongly, I suspect that countries, including ourselves, didn't expect Ukraine to stand up to Russia so well, therefore this is more out of necessity.
To not support them when they're actually giving the Russians a bloody good fight would be like burning votes.