kebabking
Not a Girly Swot, but I like them....
Well technically they don't because Ukraine's not in NATO. That's why Putin thinks he might be able to get away with it.
It's all a bit amorphous.
While Ukraine isn't in NATO, individual NATO members can choose to extend their nuclear umbrella to whomsoever they chose - so Australia, Taiwan for example are both widely considered to be under the US nuclear umbrella - there's also that while a nuclear strike might happen in Ukraine, it could easily be argued that the radiological attack had taken place against any state where the fallout landed - and based on where it went after Chernobyl - that's a lot of NATO states.
I think the big thing is that Putin, and the wider Russian power/cultural structures, thinks he can because everyone else is afraid of Russia, and in the end will back down.
The rest is technicalities, what matters is the big theme.