Answer by John Lively:
This man, Stanislav Petrov, literally saved the World.
He was in command of a Soviet missile silo back in 1983. His job was to watch for a potential nuclear attack from the United States, and if necessary – to respond with the power of God.
On September 26th 1983, he received a missile warning – alerting him that a potential nuclear ICBM (Inter Continental Ballistic Missile) strike had been launched against his country.
Now, as a good Soviet officer – when this alarm was raised, it was Petrov's job to relay this to his high command immediately, which most likely would result in a full-scale counter strike on the part of the Soviets, the obvious result of that would have been something like…
Now, luckily for us – Mr. Petrov decided that it was more likely his new warning system was faulty, rather than the U.S. launching a pre-emptive nuclear strike. In addition, the fact that his warning indicated five missiles in the air seemed strange, because all intelligence suggested any U.S. strike would be "all-out", that is, raining nukes, so "only" five seemed strange.
It turns out our friend's intuition was right – the malfunction, that had nearly caused WW3, and the deaths of countless millions, was actually caused by the warning system detecting clouds:
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
That's about as close to saving the World as one man is ever likely to get in my book…
(I certainly didn't think this would be my first 1k upvote answer, but thanks for reading and upvoting, and thanks to Mr. Petrov for saving the World!)