NATO doctrine is basically air superiority against any invading force, with the ability to wreak destruction far behind the front lines.
Conveniently the Iran war has depleted stockpiles of almost everything.
The reality is NATO is vulnerable on two fronts.
The first is that NATO has no defences against the kind of drone and missile waves Russia has been using against Ukraine. A surprise attack could easily take out a large part of NATO's air superiority and do significant damage to arms factories.
The second is more serious - capture of the independent nuclear deterrent. The US is clearly giving up on defending Europe, the UK's deterrent is barely functional, and only France has a truly independent deterrent.
Russia has spent a lot of time and money trying to get a puppet government elected on France, along the lines of the governments in Hungary, Slovakia, and the US.
If France stops being a deterrent Russia would be able to nuke Brussels - and perhaps a few other capitals to make the point - and likely force immediate surrender.
The question is really whether Russia can hold on until the French elections next year.
Ukraine isn’t under anyone’s nuclear umbrella, but Russia hasn’t done more than threaten to use nuclear weapons in that war. Probably because it’s not at all clear that it would actually force a surrender.