Early on the morning of Sept. 26, 1983, Stanislav Petrov helped prevent the outbreak of nuclear war. A 44-year-old lieutenant colonel in the Soviet Air Defense Forces, he was a few hours into his shift as the duty officer at Serpukhov-15, the secret command center outside Moscow where the Soviet military monitored its early-warning satellites over the United States, when alarms went off.

Computers warned that five Minuteman intercontinental ballistic missiles had been launched from an American base. “For 15 seconds, we were in a state of shock,” he later recalled. “We needed to understand, What’s next?”

After five nerve-racking minutes, Colonel Petrov decided that the launch reports were probably a false alarm. He made the decision to report the alert as a system malfunction.

As he later explained, it was a gut decision, at best a “50-50” guess, based on his distrust of the early-warning system and the relative paucity of missiles that were launched.

Had he followed protocol, retaliatory strikes would surely have followed, triggering an American response in kind and mutually assured destruction.

The false alarm was apparently set off when the satellite mistook the sun’s reflection off the tops of clouds for a missile launch. The computer program that was supposed to filter out such information had to be rewritten.

In 2013 he was awarded the Dresden Peace Prize. He was the subject of a 2014 hybrid documentary-drama, “The Man Who Saved the World.”

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