Nuclear Holocaust

It is possible, is it plausible?
Obviously, the United Nations, and practically every single country in the world, is aware that a nuclear war would most likely result in a nuclear holocaust, or perhaps just causing a large portion of the earth's land mass inhabitable. So the only likely scenario where a Nuclear Holocaust could happen is when hostile extremists gain access to WMDs.

I'm interested in your opinion. Of course, United Nations is well prepared for situations such as these, but I want to hear the arguments and counterarguments you guys may have for a nuclear war of sorts.

PS: I said "most likely cause a nuclear holocaust" because as of right now only major powers have wmds. Those major powers have stronger alliances with some than they do with others.

A nuclear holocaust would be the systematic killing of a certain group by means of nuclear weapons. That's not what you're talking about here.

I think you meant something more along the lines of a nuclear Armageddon of sorts. I think its plausible. All they need is some computer software to go bad and were all dust.

"Destruction or slaughter on a mass scale, especially caused by fire or nuclear war"

"Nuclear holocaust or nuclear apocalypse refers to a possible complete or nearly complete annihilation of human civilization by nuclear warfare."

I believe that there are some nations (including the USA) who have missiles powerful enough to destroy entire countries. Even look into it. There's secret underground "cities" that can support life and they are building more. Seems to me like they're preparing a place safe to repopulate if there is such an attack. Obviously this place would be off limits to general public and only for the important people.