On Sept. 26, 1983, Lt. Col. Stanislav Petrov of the Soviet Air Defense Forces received a warning that the U.S. had launched an ICBM toward the Soviet Union. He dismissed it as a false alarm. Later four additional missiles were detected, and again Petrov decided they were phantoms.

He was right, but he couldn’t have been certain, and if he’d followed protocol he might have started a full-scale nuclear exchange between the superpowers. Bruce Blair of the World Security Institute said, “I think that this is the closest we’ve come to accidental nuclear war.”