Neither the U.S. or Russia has a clear idea of when the other side might use nuclear weapons, but both sides believe their position is clear. From Scott Ritter at consortiumnews.com:
We are, literally, on the eve of destruction. Now is the time for the kind of political maturity leaders rarely demonstrate.
Wars should be avoided at all costs. Nuclear conflict should never be contemplated.
These two truisms are often spoken, but rarely adhered to. Wars occur all too frequently, and so long as nations possess nuclear weapons, their use is contemplated on a continuous basis.
The ongoing Ukrainian-Russian conflict has put the world’s two largest nuclear powers on opposing sides, with the U.S. supporting a Ukrainian military that has become a de facto proxy of NATO, and Russia viewing its struggle with Ukraine as including the “collective West.”
Since the initiation of Russia’s “special military operation” in Ukraine, both the U.S. and Russia have played their respective nuclear cards.
Russia has made it clear that any intervention by NATO would be considered an existential threat to the Russian nation, thereby invoking one of the two clauses in the Russian nuclear posture in which nuclear weapons could be used. (The other would be in response to a nuclear attack against Russia.)
The U.S. has made it clear that any attack by Russia against a NATO member would invoke Article 5 of the NATO charter (the “collective defense” clause), resulting in the totality of the alliance’s military capabilities, including nuclear weapons, being made available in response.