ie. there are still diplomatic ways out of this open to Putin. If he is not open to considering them and feels cornered, it is his own shortsightedness and failing.

It has been said that you should never draw your pistol unless you are going to shoot. I don't know if that's true; but I do think it's unwise to threaten violence unless you are quite sure you are prepared to carry through.

I think Putin is prepared to carry through (he's made the threat more than once). Russia seems to be losing in Ukraine; but they can't be seen to be slinking away with their tail between their legs. So I think there's a good chance Russia will pop a tactical nuke in Ukraine.

For the same reasons, NATO can't be seen to tolerate the popping of tactical nukes in a regional conflict; they would have to retaliate, for fear that their nuclear deterrent be treated as a joke. I haven't any idea how NATO might select their target, though; striking a city would be gross over-reaction, and striking Russians in Ukraine would be rather counterproductive.

NATO should definitely not respond to nukes with nukes unless Putin drops one on a NATO country. Ukraine is not in NATO. That makes it a first strike, not a retaliatory strike. Putin's propaganda has been that NATO is an aggressor threatening Russia. If NATO strikes Russia in retaliation for a strike on a non-member country that only vindicates him, and you know, leads directly to WWIII

Hey, I didn't mean to sound like a cheerleader for global thermonuclear war! I'm just observing that the logic of nuclear deterrence calls for a response whoever Russia attacks. If it becomes clear that Russia can nuke anyone it likes as long as they're not in NATO, then it can march into e.g. Georgia or Armenia with a company of conscripts, and take over, declaring that any resistance will be met with nukes.

Suppose Russia pops a small nuke in Ukraine? Really, that's not going to achieve any major military victory. They'd need a few to end the war in Russia's favour. That would result in clouds of fallout afflicting NATO countries. At some point you'd have to say "that's enough". But at what point? After two? A dozen? Only after the first megaton city-flattener?

I was a member of CND for a year, back in the 80's. I still favour unilateral disarmament. I quit not because I didn't support the goals, but because I was shocked at the political infighting I observed at the Annual General Meeting.