Why don't we just say "we don't want you to invade Ukraine and if you do we will use our nuclear weapons..."? Isn't that what they're supposed to be for? I mean America et al are massing forces in the region, so they must be planning for a possible military intervention.
On the point of troops, they're not really massing troops if you consider the actual size of the deployment and location. It's more of a token gesture.
We're talking about a few thousand troops in Poland, moving one thousand to Romania and NATO itself has less than five thousand in the Baltic states.
For context, there are twenty-five thousand (ish) US troops stationed in the UK alone and the actual number moving from the US and increased readiness is tiny.
I think it's more of trying to reassure states, moving certain capabilities (intelligence, perhaps) and providing a bite to the press rather than anything substantial.
In terms of nuclear weapons, I get the point about using the deterrent, but in reality saying it about Ukraine would in the long-run make their use more likely.
Without wanting to simplify the MAD theory too much, you want to have nuclear weapons to make their use more less likely; if it becomes more likely, it's failed.
To consider their use it has to meet a threshold and anything below that is dealt with in other ways.
