So if a land invasion by foreign troops is going to be interpreted as no big deal, why would the fall of Poland matter from a nuclear standpoint? You can see it building in the public discourse as people convince themselves that they can push to an absurd distance without Russia being able to respond - we've seen their border integrity violated, strikes on Moscow, British missiles being lobbed in to Russian territories, an explicit US strategy of bleeding them out and fairly credible calls for regime change where it isn't obvious how far the US is acting on the idea.
If we assume by symmetry that the US would take similar abuse before it launched any nukes, what on earth is supposed to happen in Europe to provoke a serious response? I wouldn't chance it, but it looks a lot like they're bluffing a willingness to do anything and some idiot dressed up as a general will one day will do something really stupid. We're exploring for the line where Russia breaks the nuclear taboo here; as far as I remember the game-theoretic response to salami tactics is supposed to be wildly disproportionate aggression. And there is a decent chance that it'll go unpunished since the US is clearly not going to stand up for Ukraine to the point where it risks blowback to the US proper.