It would've taken one launch officer who wasn't right in the head to trigger a nuclear war and start World War III. For nearly two decades, the nuclear launch code was "000000000000," according to Dr. Bruce G. Blair in his 2004 article "Keeping Presidents in the Nuclear Dark."
In the documentary Countdown to Zero, Dr. Blair describes the launch sequence and the device into which the code was entered:
When I was serving in the Air Force as a launch officer there was a device in the launch control center into which 12 digits had to be dialed in to unlock the missiles from firing. This had been installed under Robert McNamara over the objections of the Strategic Air Command. Since they couldn't prevent the panel from being installed the strategic air command in Omaha had set these codes to zero and we all knew it. That was the secret unlock code for firing our missiles, twelve zeros. In fact in our launch checklist we had to ensure that the unlock code was set to all zeros before we completed the launch sequence.