It would kill hundreds of millions within 72 minutes and up to 5 billion people within a year. And also concepts like "Launch on warning" and "Sole presidential authority".
This is hard to watch but it explains what happens the second an enemy ICBM's exhaust is detected.
These systems are built solely as deterrents they make no sense as weapons of war.
Have you read "On the beach" by Nevil Shute..? I have a weakness for classic dystopias* - this one covers the aftermath of a global thermonuclear war. I'm going to out myself here as a gaylord, but it's one of the few books that ever brought me close to tears by the sheer senselessness of it all...
[*Irrelevant tangent warning: for years I hadn't realised that my English Teacher had primed me for this by the books and plays we worked through or he recommended: Lord of the Flies, The Death of Grass, Trillions, When the Wind Blows, Death Dolls of Lyra, Brave New World, 1984, Animal Farm, God the list goes on and on... yet he was the most optimistic madman I ever met! Great fella]