Established History

As acknowledged by the Infinity Doctor, the Time Lords were sworn to protect Established History. The Rutan Host denounced this concept as being no different than the "arrogant, misguided" Sontaran assertion of Established Space. (PROSE: The Infinity Doctors)

The Sixth Doctor noted that, had the Neomorph Cybermen succeeded in diverting Halley's Comet to destroy Earth in 1985, it would bring about "a massive change in established history". Knowing that the Time Lords would never allow it, he came to realise that he had become their unwitting agent in thwarting the plot on Telos. (TV: Attack of the Cybermen) The Infinity Doctor also cited the Cybermen as having threatened established history, that the consequences to universal history would be catastrophic were they to destroy the Earth before the human race had reached their full potential. (PROSE: The Infinity Doctors)

Billions of years after the fact, the Eleventh General told Clara Oswald, who had been extracted from the very end of her time stream, that her death was an established historical event and could not be altered. He also warned the Twelfth Doctor that, having been dead for "half the lifetime of the universe", an attempt to change Clara's death could fracture time itself. (TV: Hell Bent)