Many countries also have nuclear weapons, but they have not been used in combat since WWII. The big deterrents would be that any detonation within the continental US would likely have a detrimental impact on the state who launched it, and that any retaliation would result in mutual destruction.
The premise is that the 50 states are at war with each other. It's hard to imagine the reasons for such a situation in the first place. If NM were losing to Texas on the south and Colorado on the north and the population being massacred then perhaps the modern day Curtis Le May - or Jack D. Ripper - would say it's a justified nuclear war since they are being destroyed anyway.
That lack of a real justification for the war in the first place is what makes it hard to have any meaningful response to the topic.