There are actually two. Pakistan and India for reasons as rational as attacking the hotel in Mumbai.
And the other, is that we engage in a direct land war against Russia or a large scale air war.
It’s a huge blind spot that we do not understand how resolved they are that no enemy will ever visit the disasters of the past on them without facing ruin on their homeland in return.
Those are the two I see. Pakistan/India with God knows what spark, or US/Russia as a result of a miscalculated belief that a direct conventional fight is in the cards.
No government is ever going to shoot a nuke first. There is no scenario for that. Only an act of terrorism or an accident will a nuke be launched/detonated. And since that type of launch or detonation will be a singleton it will be apparent it is not a general nuclear strike.