Putin is basically daring other countries to intervene, threatening to resort to nukes. He has to know that no one wins if nukes come into play. His country will not survive. In some ways, I wish someone would call his bluff, in others, I of course worry he just might be serious. But I do think he's bluffing, feigning strength for his people. The last thing he wants is to look weak.
I do think he is bluffing as well. That he has to already resort to this bluff means things are going ****** for him.