In case you didn't get that memo: the shit in the Ukraine didn't start with Russian involvement.
It started with very active meddling, spending copious amounts of money, into the internal politics of the country. It should also be noted that much of that money went to ultra-right-wing militant organizations, who have zero respect for democracy or freedom. Essentially, the USA bought itself a (fascist) private army to take control (or at least try to) over the natural resources in East Ukraine and Crimea. Russia responded rather predictably to that.
Some might even argue that they had every right to do so. I'd love to see Americans react, if Russia were to sponsor some kind of anti-US militia in Alaska, that would subsequently take control of state (military bases with missile launch facilities included). I'm pretty sure that US citizens would call that for what it is: an act of aggression.
No, the reason Russia doesn't invade Poland or the Baltic countries isn't because of NATO. Ironically, if those countries are at risk in any way, it's probably rather because of NATO. NATO (and its members) keeps insisting that the organization is defensive in nature only, yet its actions tell a rather different story (and it has done so for a long time, at least since the '90).