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 think the better question would be if China crossed the Yalu River today and joined the North Koreans to assault South Korea, what would the response of the US, South Korea, and Japan be? The latter two can trivially develop nuclear weapons if the US security situation degrades.
I think wielding nuclear weapons requires a baseline level of competence to exploit.