On September 26, 1983, just three weeks after the Soviet military had shot down Korean Air Lines Flight 007, Petrov was the duty officer at the command center for the Oko nuclear early-warning system when the system reported that a missile had been launched from the United States, followed by up to five more. Petrov judged the reports to be a false alarm, and his decision is credited with having prevented an erroneous retaliatory nuclear attack on the United States and its NATO allies that could have resulted in large-scale nuclear war. 