Widely assumed: If a nuclear power felt itself "sufficiently threatened", then any nice-sounding things they had previously said about "use is unacceptable" might be - OOPS! - forgotten in the heat of the moment.

Nuclear powers mostly avoid conventional wars with each other because: Actual victory (which is presumably the Big Payoff being sought, when a country goes to war) would be too likely to suddenly turn to ashes, because the other power felt "sufficiently threatened" and started using nukes.