The event that created the Kelvin timeline.
In 2233 the USS Kelvin was on an ordinary mission when it encountered an anomaly in space, a lightning storm that gave way to a gigantic Romulan mining vessel, the Narada. Its commander, Nero, had come from the future with a grudge and with powerful weapons that made the Federation ship look primitive.
Nero demanded that the Kelvin's captain come aboard as a prisoner. Captain Robau was killed during the interrogation, leaving First Officer George Kirk in command. He ordered the crew to evacuate in shuttles, including his pregnant wife Winona, and stayed on the bridge to steer the crippled ship into the Narada, buying the survivors time.
George Kirk died as his son James was being born, and this event changed history. The timeline diverged, so that the Kirk who grew up in this reality was raised without his father, and the people who would form the crew of the Enterprise met under very different circumstances. Nero's attack set in motion the events leading to the destruction of Vulcan in 2258.