Via Bruce Schneier, the author of How the End Begins describes how no one can ever be absolutely certain an order to destroy civilization is authentic:
Can the president start a nuclear war on his own authority—his own whim or will—alone? The way Brigadier Gen. Jack D. Ripper did in Dr. Strangelove? What if a president went off his meds, as we'd say today, and decided to pull a Ripper himself? Or what if a Ripper-type madman succeeded in sending a falsely authenticated launch order? You're about to kill 10 million people, after all.
Anyway, back down there in your launch capsule you might allow yourself to wonder: "This launch order, is this for real or for Nixon's indigestion?"
If you were asking yourself that question, you wouldn't be the only one. James Schlesinger, secretary of defense at that time, No. 2 in the nuclear chain of command, was reported to be so concerned about Nixon's behavior that he sent word down the chain of command that if anyone received any "unusual orders" from the president they should double-check with him before carrying them out.
So there you are, having just received the order to launch nuclear genocide. Should you suppress any doubts, twist your launch key in the slot simultaneously with your fellow crewman and send death hurtling toward millions of civilians halfway around the world? Without asking questions? That's what you're trained to do, not ask questions. Trainees who asked questions were supposed to be weeded out by the Air Force's "psychiatric consideration of human reliability" requirement. I've read this absurd Strangelovian document, which defined sane and reliable as being willing to kill 10 or 20 million people with the twist of a wrist, no questions asked.
Oh, yeah, I'll sleep well tonight.