On 26 September 1983, the nuclear early warning system of the Soviet Union twice reported the launch of USAF Minuteman intercontinental ballistic missiles from bases in the United States. These missile attack warnings were correctly identified as a false alarm by Stanislav Yevgrafovich Petrov, an officer of the Soviet Air Defence Forces. This decision is seen as having prevented a retaliatory nuclear attack based on erroneous data on the United States and its NATO allies, which would have probably resulted in nuclear war and the deaths of hundreds of millions of people. Investigation of the satellite warning system later confirmed that the system had malfunctioned.
Shortly after midnight, the bunker's computers reported that one intercontinental ballistic missile was heading toward the Soviet Union from the United States. Petrov considered the detection a computer error, since a first-strike nuclear attack by the United States was likely to involve hundreds of simultaneous missile launches in order to disable any Soviet means of a counterattack. Furthermore, the satellite system's reliability had been questioned in the past. Petrov dismissed the warning as a false alarm, though accounts of the event differ as to whether he notified his superiors or not after he concluded that the computer detections were false and that no missile had been launched. Later, the computers identified four additional missiles in the air, all directed towards the Soviet Union. Petrov again suspected that the computer system was malfunctioning, despite having no other source of information to confirm his suspicions. The Soviet Union's land radar was incapable of detecting missiles beyond the horizon, and waiting for it to positively identify the threat would limit the Soviet Union's response time to a few minutes.
It was subsequently determined that the false alarms were caused by a rare alignment of sunlight on high-altitude clouds and the satellites' Molniya orbits, an error later corrected by cross-referencing a geostationary satellite.
In explaining the factors leading to his decision, Petrov cited his belief and training that any U.S. first strike would be massive, so five missiles seemed an illogical start. In addition, the launch detection system was new and in his view not yet wholly trustworthy, while ground radar had failed to pick up corroborative evidence even after several minutes of the false alarm.