I think if Russia drops a nuke on Ukraine then even China will desert them.
India for sure will stop trading with Russia, lest it be seen to condone such insanity (India has a nuclear armed rival next door-India will not want Pakistan get any ideas).
I think this is the only reason Russia did not nuke Ukraine.
Well, yes. Stop trading with them? I can believe that.
I don't believe for a second that even a single country will openly declare war on Russia over it or attempt to nuke Russia in return.