Jayjay said:
So, let's say Russia does use a "tactical" nuke on Kyiv. City wiped out, including Zelensky and most of Ukrainian leadership (I expect the military to be still able to function though, being non-centralized).
What happens next? Does the NATO or US respond in kind with a nuclear strike on Russia? Or will they finally enforce a no-fly zone in Ukraine and actively push Russian troops out, knowing it may lead to more nukes?
I don't know, but why would Russia use tactical nukes in that manner?
In fact, Kyiv is an extremely unlikely target, for several reasons. Just for example:
1. Putin had the ability to target Zelensky and likely kill him, as he chose to stay openly in Kyiv, in a known location. He chose not to do so. Why? I don't know. Maybe he reckons targeting leaders is off-limits, lest he be personally targeted. But nuking Kyiv would have that effect.
2. Russia denies targeting civilians. Targeting a city would be obviously a case of targeting civilians.
3. They're now trying to get the Donbas region, not all of Ukraine. They've limited their territorial ambitions.
Here's a more realistic scenario, I think: Russia would only use nukes if they failed to win without them, and in that case, they would target front line troops first, not with a nuke but with dozens of them, several times - it would one barrage of nuclear rocket artillery after another, using very low yield nukes (their tactical nukes can very likely set the yield before firing).
What happens next? Chances are Russia takes the Donbas region using conventional weapons in cities, as Ukrainian forces have been severely weakened and can't reinforce the defenders.
But if that's not enough, then Russia can tell Ukrainian forces to surrender. If they do not, then it can tell them to leave cities, towns, etc., and get in the open, accusing them of using civilians as human shields if they stay put, or something along those lines. If the Ukrainian remain, Russia can use conventional weapons against those forces, who cannot easily be reinforced easily after the heavy losses resulting from nukes.
Now, if that is not enough, Russia can attack Ukrainian troops in civilian areas with chemical rather than nuclear weapons, so that they tell the Russian population it's the Ukrainian Nazis targeting Ukrainian civilians to blame Russia, while using the tactical nukes outside populated areas.
If even that is not enough to win, then Russia would consider further escalation, but it's hard to see how Ukrainian defenders could withstand half of that; they've proven much stronger than anticipated so far, but there is so much they can take.
What would the US and NATO do in the above scenario?
I do not know for sure, though I'm pretty sure one thing they would not do is resort to nuclear weapons. One probable option is impose further sanctions on Russia. Even stop buying gas (including Germany), despite the civilian hardships. They could use Russia's nuclear attacks to get more countries to impose sanctions, further isolating Russia, in order to increase the chances that someone will just kill Putin and replace him in their own interest. And of course, they'd give even more weapons to the remaining Ukrainian defenders.
How would it continue?
I do not know. But that's still a good distance from global nuclear war. Russia has further military options. And Biden is not willing to sacrifice the existence of the US or at least tens of millions of US citizens over Russia's nuclear attacks on Ukraine. The same goes for Johnson and Macron.