This article by Nicholas Thompson in Wired is titled “Inside the Apocalyptic Soviet Doomsday Machine‘. I excerpt this:
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What: Midway through the Cold War, American leaders began to worry that a rogue US officer might launch a small, unauthorized strike, prompting massive retaliation. So in 1962, Robert McNamara ordered every nuclear weapon locked with numerical codes.
Effect: None. Irritated by the restriction, Strategic Air Command set all the codes to strings of zeros. The Defense Department didn’t learn of the subterfuge until 1977.