The story, as I remembered it, and as it was since repeated many times in the press, is that in the 1960s, Secretary of Defense Robert McNamara wanted an extra layer of security against unauthorized or accidental launch, but that the Air Force decided that the best thing to do was to ignore McNamara — or more precisely, to follow the order to create a launch code but then set it at a line of zeros. 

I said I doubted the story, since the only source I could find for it was Bruce Blair, who mentioned it in a 2004 article. I theorized that Blair (a former missile officer himself and a leading arms control analyst now at Princeton) had either been misunderstood, or had been misleading: perhaps, I thought, he meant that the initial settings on the mechanism for the codes had to be set at zero.