Indeed. But the bully also knows that using nukes would be the end of him, even if he gets to take everybody else along. Russia would absolutely use nukes if it's actually invaded, but a conventional military confrontation on foreign territory is a very different matter.

As for being willing to take more pain - you're right, that's a big part of the problem for the West. But it needs to understand that, just like in 1938, avoiding the bully only delays the inevitable, and the pain will still come eventually.

I worry that it's not about one big red button but gradual escalation.

Somebody decides that using tactical kiloton Nukes is the way to go and other side would not dare escalate further.

A lot of people think if they do the big fearsome escalation, then the other side will back off. Everything up to this point indicates this is Putin's logic. He may be over committed now - what option other than finishing invasion does he realistically have that preserves any goals or prerogatives? In some ways the rational thing, from his perspective, is to keep escalating until other side acts "rationally" and backs off.

It's all a giant game of chicken, and I am not convinced that all state leaders anywhere let alone everywhere are rational and fearful enough.