Not acting on this would completely destroy NATO and also permanently destroy US credibility on the world politics and in terms of military power. Realistically speaking NATO and US could not afford not to go to a war or at least armed conflict with Russia.
Granted Russia is still seen as an adversary, but if Russia manages to quickly steamroll the Balkans/Poland/Ukraine and then challenges NATO to do something about it, I'm not sure there's the stomach for a bloody, protracted invasion of the continent to reclaim the lost members.
There's also a large swath of the US that sees NATO as a waste of US time/money, with members that largely don't pull their weight and are just mooching off the US, hell that was part of Trump's platform. And Secretary Gates under Obama complained, more diplomatically, about the same thing.
There's a substantial chance that a Russian invasion of any part of Europe would be seen as a "European problem" by a large chunk of Americans. None of this is to say I support that position, but that's where we're at IMO. America is largely turning inward for the first time since WWII, the question is whether Europe will figure that out before Putin does.
How about pay your fucking debt and keep Russia at bay.