US nuclear doctrine is literally built around anticipating the possibility of an attack by a nuclear weapons state against a non-nuclear NATO ally. That is why it is different from the Russian nuclear doctrine. The US would not be existentially threatened by Russia deciding to nuke the city of Riga, but since US nuclear doctrine allows for first use in the context of deterrence, anyone planning on bombing Riga now has to consider the likelihood of US retaliation. That is how nuclear doctrines work. They aren't just an excuse for insecure leaders to show big dick energy, they are a public record of the lines that should be avoided at all costs.
If the Russian government decides to just ignore US nuclear doctrine because Latvia really needs to be part of the Russian empire again, then that falls into the category of insanity or incompetence. This isn't the cold war, and the US isn't going to launch all its ICBMs because some shitty rusted out BTR rolled over the border into Estonia, but the line has been drawn. If Russia's leadership chooses to cross the line knowing it could end human civilization, it once again raises the question of why they are running a country instead of being sectioned in a psychiatric ward (although I guess the real answer there is that they aren't politically advocating for the rights of persecuted racial minorities).