Earlier quoted context omitted.
MAD precludes total wars of national survival ala WW2. It does not at all preclude the sorts of cabinet wars that were fought between the European powers for centuries before Napoleon.
If we crossed the Yalu River today, do you think we'd be alive tomorrow to talk about it? Nuclear deterrence, as part of the taboo, prevents any nation from violating the sovereignty of a nuclear power. The only exception I know of is the Yom Kippur War, and if there is ever a person who never received a well-earned Nobel Peace Prize, it's Golda Meir.
I still wouldn't want to be in Seoul or Tokyo if that happens though, and yes, there's a non-zero risk somebody somewhere gets trigger happy and things spiral out of control.