NEW YORK — 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 had begun 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 a US base.

“For 15 seconds, we were in a state of shock,’’ he later recalled. “We needed to understand, ‘What’s next?’’’

The alarm sounded during one of the tensest periods in the Cold War. Three weeks earlier, the Soviets had shot down a Korean Airlines commercial flight after it crossed into Soviet airspace, killing all 269 people on board, including a congressman from Georgia. President Ronald Reagan had rejected calls for freezing the arms race, declaring the Soviet Union an “evil empire.’’ The Soviet leader, Yuri V. Andropov, was obsessed by fears of a US attack.

Colonel Petrov was at a pivotal point in the decision-making chain. His superiors at the warning-system unit reported to the general staff of the Soviet military, which would consult with Andropov on launching a retaliatory attack.

After five nerve-racking minutes — electronic maps and screens were flashing as he held a phone in one hand and an intercom in the other, trying to absorb streams of incoming information — Colonel Petrov decided that the launch reports were probably a false alarm.

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.

Colonel Petrov died at 77 on May 19 in Fryazino, a Moscow suburb, where he lived alone on a pension. The death was not widely reported at the time. It was confirmed by his son, Dmitry, according to Karl Schumacher, a political activist who, after learning in 1998 of Colonel Petrov’s Cold War role, traveled to Russia to meet him and remained a friend. The cause was hypostatic pneumonia.

Historians who have analyzed the episode say that Colonel Petrov’s calm analysis helped avert catastrophe.

As the computer systems in front of him changed their alert from “launch’’ to “missile strike,’’ and insisted that the reliability of the information was at the “highest’’ level, Colonel Petrov had to figure out what to do. The estimate was that only 25 minutes would elapse between launch and detonation.

“We knew that every second of procrastination took away valuable time, that the Soviet Union’s military and political leadership needed to be informed without delay,’’ he told the BBC. “All I had to do was to reach for the phone; to raise the direct line to our top commanders — but I couldn’t move. I felt like I was sitting on a hot frying pan.’’

As the tension in the command center rose — as many as 200 pairs of eyes were trained on Colonel Petrov — he made the decision to report the alert as a system malfunction.

“I had a funny feeling in my gut,’’ he told The Washington Post. “I didn’t want to make a mistake. I made a decision, and that was it.’’

Colonel Petrov attributed his judgment to both his training and his intuition. He had been told that a nuclear first strike would come in the form of an overwhelming onslaught.

“When people start a war, they don’t start it with only five missiles,’’ he told the Post.

Moreover, Soviet ground-based radar installations — which search for missiles rising above the horizon — did not detect an attack, although they would not have done so for several minutes after launch.

Colonel Petrov was at first praised for his calm, but in an investigation that followed, he was asked why he had failed to record everything in his logbook. “Because I had a phone in one hand and the intercom in the other, and I don’t have a third hand,’’ he replied.

He received a reprimand.

Colonel Petrov had largely faded into obscurity when his role in averting nuclear Armageddon came to light in 1998 with the publication of the memoir of General Yuriy V. Votintsev, the retired commander of Soviet missile defense.