That's a good point, i read this thread top to bottom because this topic has always been fascinating to me. Others have brought up how war wouldn't go from 0 to missiles flying immediately, there would be a crisis and build-up period. So I can see something like that ability to mass produce nuclear weapons and demonstrate it acting as a deterrent and therefore be useful.I can't think of an instance off the top of my head where a massive arms build up caused the other side to back down though.

To your specific point, I think if Ukraine had the keys to even one nuclear weapon and Russia believed they'd use it the invasion would never have happened in the first place.