MAD is there for a reason, anyone who thinks that a nuclear attack within territory of a NATO country or forces within the areas that fall under Article 5 won’t trigger a nuclear response is kidding themselves.

And not only that eroding the threat of MAD makes the world a far dangerous place because it makes a first strike more likely.

That said even if the conflict goes nuclear there are different levels of escalation a targeted nuclear attack on say a military target would likely trigger a proportional response even if it’s a nuclear one.

And outside of NATO the US would very likely respond with a retaliatory nuclear strike against a nuclear strike on its forces where ever they might be.