Just after midnight on 26 September 1983, in a bunker outside Moscow, a Soviet early-warning satellite reported that the United States had launched a missile, and then that it had launched five. The officer on duty, Stanislav Petrov, was meant to pass the warning up. He reported a fault instead. A real first strike, he reasoned, would not open with five, and the ground radars still showed nothing. He was right: the satellites had taken sunlight glinting off high cloud for rocket fire. What I keep turning over is the shape of the thing he did. It was a message not sent, and its whole result was a night that carried on like any other. You can visit a battlefield. Nobody can stand in the war that didn’t happen, so the best thing he ever did left nothing behind to point at, except an ordinary morning for everyone who never knew.