That was because, IIRC, Congress mandated a launch code be added to the system, so the military added one and set it to all 0's as a loophole, because their main concern was to make sure the system would work as intended. Not having a launch code when you need it would be a bad thing. None of this precluded the existing, strict, chain of command, which included the famous "biscuit" that the president needs to launch missiles. And I believe the whole thing also needs to be authenticated by the Sec. of Defense.