On Sept. 26, 1983, Stanislav Yevgrafovich Petrov saved the world. An officer with the Soviet Air Defense Forces, he oversaw a computer-controled network of satellites that monitored the skies for a possible nuclear strike from the United States. On that day the computer system reported five incoming missiles. Petrov double-checked the data: The probability of a nuclear attack was 100%.

The system was new and Petrov didn't entirely trust it. There was no confirmation from ground radar and Petrov t...