MAD has stopped mass warfare for decades and will continue to do so.
But it didn't stop wars. The nuke powers learned how to operate and kill in large numbers without crossing the triple red line. While we were bombing Vietnam, the Soviet ships were making deliveries to Haiphong. Just an example.
If Russians actually enter Ukraine, the decision to go nuclear would belong to the US. The insanity of this move is beyond insane: The US ending civilized life as we know it over a totally irrelevant piece of real estate aka Ukraine.
I will go further...The US would not use nukes to defend Poland, either. All that nuclear talk is good for high school debates.
Imperial conquest has always entailed huge costs for average Russians:
Just like the US playing word's dumbest cop entailed a huge cost for the average American. Did you know that the US has been at war with somebody 90% of its existence? As opposed to Russia, not even close to home. In fact, we like wars 10,000 miles away as more tolerable to the average American. Maybe it's time for one in Iowa.