How close is humanity to destroying itself?

Humanity has come startlingly close to destroying itself in the 75 or so years in which it has had the technological power to do so. Some of the stories are less well known than others. One, buried in Appendix D of Toby Ord’s splendid The Precipice, I had not heard, despite having written a book on a similar topic myself. During the Cuban Missile Crisis, a USAF captain in Okinawa received orders to launch nuclear missiles; he refused to do so, reasoning that the move to DEFCON 1, a war state, would have arrived first. Not only that: he sent two men down the corridor to the next launch control