In 1983, the world came closest to a full scale nuclear war between Russia and the US.
Back then, Russia's nuclear early-warning system reported that a missile had been launched from the United States, followed by up to five more. Stanislav Petrov, the duty officer at the command center at that time, was tasked with observing the satellite early warning network and notifying his superiors of any impending nuclear missile attack against the Soviet Union. If notification was received from the early warning systems that inbound missiles had been detected, the Soviet Union's strategy was an immediate and compulsory nuclear counter-attack against the United States.
Petrov dismissed the warning as a false alarm. He deduced that 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. His suspicion was confirmed when no missiles arrived at the proposed time of impact.
Later, it was 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.
So the only reason why civilization as we know it didn't cease to exist in 1983 was the level-headed behavior of a single man.