I just read a piece by a retired General in the UK. I don't remember his name or even where I read it so I can't provide a link.
The long and short of it is that he was of the opinion that although he understands why NATO or the US don't want to enforce a no fly zone over Ukraine, he thinks we should worry less about Putin's threat of using Nukes, except possibly strategic ones because he's not as stupid as people think and he knows that if he actually pressed the big button it would also be the end of him and Russia too.
He feels that eventually we (the west) will have to enforce a no fly zone and it basically boils down to do we want to fight Russia now or do we want to fight them later because we are going to fight them so perhaps a no fly zone would let the west give Putin a bloody nose and finish him while the Russian troops are mostly in Ukraine and in a mess.
He also felt that if Putin fired even one small strategic nuke in Ukraine, the west will immediately get it's boots dirty with Ukrainian mud.
Incidentally, I saw on CNN last night that Putin is asking Syria to send troops to reinforce his own in Ukraine.