If there is a nuclear war resulting from the Russo-Ukrainian conflict, then the chronology will be something like this:
1. Russia using tactical nuclear weapons against Ukrainian troops in newly annexed regions of Ukraine (Donbass, Kherson and Zaporizhia)
2. Americans using tactical nuclear weapons against Russian troops in these regions
3. Russians responding by nuking American bases in EU, middle-east, Japan and Korea
4. All-out nuclear war between Russia and NATO
Secondly, the United States has already stated that after Russia uses nuclear weapons in Ukraine, the Russian Black Sea Fleet will immediately be destroyed, followed by the Baltic Fleet. Then a pause will be made to assess the situation. But if NATO receives information that will indicate specific actions aimed at the use of nuclear weapons, then the alliance can activate a developed program that allows for 40 minutes to destroy any country with non-nuclear weapons, even such a large one as Russia. Putin is so intimidating the world with nuclear weapons that NATO can believe it and launch a preemptive strike, even if Russia's actions turn out to be only intimidating.