"There is absolutely no doubt in my mind that any country in the NATO umbrella being hit would lead to the United States retaliating decisively."

How?

http://defense-and-freedom.blogspot.com/2016/07/how-to-invad...

Also, do you know what the UdSSR "ax theory" was? Or the "ed-escalation" strategy of the 90ies?

Look at the map. The Baltic states can't be defended. At least not with the military that is there in the moment. Could they be taken back? Unlikely because Russia would immediately retaliate tactically (nuclear).