pretty much everyone is not stupid enough to do that, because as we've known since the 60s at least, we'd all lose. Russia has nuke subs ready to do retaliation, we have a fuckton of them too, China does, random other countries like the EU and even Israel is ready to nuke the fuck out of everyone else just in case. Putin makes weak nuclear threats and he knows that "we'll nuke you!" would result in most of Russia being completely smashed, along with most of the EU and USA. The danger is some idiot feeling nihilistic and egotistical enough to try it. It would never be a reasonable policy decision by any country.