Given that neither side has directly attacked the other yet? Almost all of them.
Using nuclear weapons against an opponent who has enough nuclear weapons to retaliate is a "flip over the chess board and stomp on the pieces" move. Presently the US isn't even playing against Russia, at best it's sitting behind Russia's opponent and whispering chess moves into their ear.
There's also gradations of nuclear exchange, including a limited exchange (i.e. not against cities) that doesn't necessarily escalate to a strategic exchange. While obviously extremely dangerous and unpredictable, some think you can skirt that line successfully in a war.
This is the first time that I have seen this terminology. I tried to Google for it, but I cannot find any information about this idea. Are there any war college studies (US/Europe) that you can share?
Controversially, I don't think generals from either the US, nor Russia, would be willing to "pull the trigger" and launch a nuclear attack. Yes, I really think there would be a constitutional crisis where senior ministers and military leaders might stage an "instant" coup to prevent a nuclear attack.
Using nuclear weapons against an opponent who has enough nuclear weapons to retaliate is a "flip over the chess board and stomp on the pieces" move. Presently the US isn't even playing against Russia, at best it's sitting behind Russia's opponent and whispering chess moves into their ear.