I think NATO would, under some circumstances, engage militarily, but not in this conflict. If, for example, a small non-NATO country with no nukes was to attack a member state, NATO would respond pretty quickly. In an alternate universe, if Ukraine (without any support from Russia) was to attack Poland, NATO would respond. But NATO is not strong enough to oppose Russia without major casualties. I believe that if all NATO member states were to at once attack Russia today, they would win but it would be a war, not a surgical strike. This is why we won't see that happening.