The US would not go to war over Estonia but it has nothing to do with nuclear weapons. It just wouldn't be worth the death from a conventional ground war. Additionally Estonia would be kind of responsible for bringing it on themselves via terrible policies.
As to nuclear weapons, we spend too much time discussing them. Nuclear weapons do not deter military action at all. If Ukraine had kept their nukes, Russia would still have taken Crimea. The only thing nukes do is deter other people from using their own nukes.
1. Allowing a Russian occupation of parts of Estonia would jeopardize the credibility of NATO and all your other military alliances. The size and economic importance of Estonia doesnt matter in this context.
2. Do you have even a shred of evidence to back up that claim?
So you really think if Russia had taken Crimea and then Ukraine nuked Moscow, public opinion would side with Ukraine? No, it would still be condemned by almost everyone as disproportionate and a crime against humanity. That's why Ukraine would never have even considered it. Moscow knows this and that's why it wouldn't have deterred them. They know the West in general is too civilized to ever use nuclear weapons first. They can do anything they want short of nuking people and not worry about getting nuked. The only thing they have to worry about is provoking a ground war and we're not going to start one over Estonia.
Maybe if France got attacked we would go to war but that's not even realistic worst case scenario.
