It is entirely possible that Russia could overrun any of the Baltic states (the current favourite amongst analysts is Latvia) in short order and there would be little that NATO could do about it, and to be honest I've always had a bit of a hunch that the eastward expansion of NATO was always intended to be a bit of a buffer zone to trade land for response time for any potential Russian westward advance.
The trickier part is estimating what NATO would do when presented with a fait accompli by Moscow, there would have to be some kind of measure undertaken by NATO in retaliation in order to ensure the continued existence of the organisation, otherwise trust in NATO will hit an all time low if it is shown that it cannot and will not defend member nations.
It's good money that any retaliatory reponse by NATO would be pointed firmly at Kaliningrad, and what Russias response would be to the invasion of what it considers sovereign Russian soil is difficult to ascertain, needless to say it would likely involve continued escalation and the longer any conflict between NATO and Russia goes on, the more likely it is that one side will pop the nuclear cork, and from there it's limited escalation until someone either turns it off or we go full exchange and that's game over.
So ultimately the response to any Russian attack on the Baltics is probably the hardest decision that any organisation will have to take, and since NATO revolves around international agreement, and you can guarantee that there will be members of NATO who will not want to go to war with Russia over the Baltics, Germany probably, Norway and Iceland too since they would be in the firing line, Turkey is an unknown quantity at this stage, but the Eastern states would all be in favour of marching to Moscow and burning down the Kremlin, in fact it would be hard to stop them.
And while NATO is still deciding what to do next, little green men would probably appear in Talinn and Vilnius...
