So you are saying putin will not use nukes if ukraine takes crimea in a conventional war..
stating that russia wont use nukes is actually more insane
The question of nuke had been discussed many time before.
First question you need to ask is what kind of nuke they want to use <5kt? Between 5 to 20 Kt? Between 20 to 40kt? Or 40kt or above?
Second question you need to ask is where are they going to use it on. In the east? north? South? or West?
Finally, what kind of response you are looking at?
1.) If they use anything less than 5kt, they may as well just use conventional weapon, it's nothing on a 5kt device, You are talking out around 10 x 10 meters blocks. 20kt to 40kt is more or less probably would be the only thing you could potentially think of, because over 40kt would risk MAD. But still 40kt would take out a 1.5 km 1.5 km block at 2psi ground brust, you would need a lot of those to have any significant effect. For example, you would need 400 of those warhead just to level Kyiv as Kyiv city limit is 839 sq kilometers. Tactically, unless you are talking about using them in Hundreds, or even thousand, it wouldn't change anything.
2.) Where are they going to use it on? Ukraine from West to East is 1250km the widest and that's counting currently Russian controlled Donbas, If we don't count Russian occupied zone, then the wide is slightly less than 1000km. So now if you drop 40kt device on anywhere East or South, it's too close to Russia, Russian controlled Donbas and Russian controlled Crimea, the fall out would affect all those. Too close north (Kyiv is only 140km from Belarus) you are going to affect Belarus. And too close to the West, you are looking at fall out going to spill over to Poland, which mean there are just a smallarea you can nuke near the central Ukraine, which have absolutely nothing in it, it will not make any different because Ukrainian troop concentration is on the North, South, West and East. All of those you really can't nuke or you will nuke Poland, Belarus or yourself.
3.) What would be NATO response? It's stupid to assume NATO would let you use nuke and do nothing, even <40kt device, they will response, it may not be a nuclear response or even incursion to Russia, but there are a few things NATO can do and will spoil the Russian. More neutral approach will be sending in NATO peacekeeper. and basically, dare the Russia to attack them to bring the entire NATO in, most likely if that happen the war will end and Russia will not be able to gain anything since that moment, and as you said, they would have to be desperate to nuke Ukraine, which mean at that point, their foothold is small.
One step up, NATO may launch a No Fly Zone like in Baghdad with the Peacekeeper in place, If that happen, Ukraine can move under NATO air protection and if Russia can't gain Air Superiority now, they are not going to when NATO Air force is involved. And effectively declare all Russian target in Ukraine as legitimate target.
Finally a less possible scenario is NATO send in ground force into Ukraine and expel the Russian with a combine NATO Air/Land campaign much like ISAF. I don't see this possibility is high tho.
The problem you failed to see is, once you used nuke, You cannot use the threat to use nuke as a bargaining chip, because you just used that. Once you did, the West would have response because well, it's no longer whether or not you will use them, but you already did, then the focus is not on the threat itself, but rather how to minimise this threat.
