I think the critical question is what would the allies do (especially the ones with nuclear weapons) if Russia actually used one. There's a lot of treaties, words spoken and on paper but nobody really knows how people would react to the idea of partaking in a nuclear war.

If they dropped a nuke on e.g. Lithuania would the French do the same knowing that retaliation would come that could wipe out most of their people and country off the map? Would any country do that for someone else?