If the Russians feel existentially threatened— and I am not talking here about whether or not that feeling is justified— they may resort to the use of battlefield, low-yield nuclear weapons. NATO forces would not be able effectively to respond with conventional weapons and would consequently counter-attack in kind.
Nobody in the Ukraine, not even president Zelensky, seems to have believed Putin would actually invade. They thought his military buildup was a bluff, a bargaining chip to use in diplomacy. But the American elite did not want diplomacy and did everything in their power to subvert it.
Let’s try to step out of Facebook World, Plato’s Cave, and look at things as they are.