On January 24, 1961, thirty thousand feet above Goldsboro, North Carolina, a B-52 carrying two 3.8-megaton hydrogen bombs, broke apart in midair.

Both bombs began arming themselves on the way down, exactly as they were built to do if their crew released them on purpose. One landed in a tree. The other buried itself in a tobacco field, having finished six of its seven steps to arm itself. A single low-voltage switch, the kind you’d buy in a hardware store, stood between eastern North Carolina and a blast 250 times the size of Hiroshima. U.S. Defense Secretary Robert McNamara admitted later that the whole thing came down to the failure of two wires to touch.

Then on September 26, 1983, three weeks after the Soviets had shot down Korean Airlines Flight 007, Air Defence Lieutenant-Colonel Stanislav Petrov sat in a bunker south of Moscow watching a screen that told him, with what his system rated “highest confidence,” that American nuclear missiles were inbound.

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