George Orwell's dystopian novel, 1984, is in many ways intended to satirize the Soviet Union, and especially the way that, during the Stalinist regime in power during 1949 when 1984 was published, Soviet revisionist historians rewrote history to support their version of Marxist-Leninist ideology, expunging from history books anything not flattering to their regime or belief system.
In the same way, the Ministry of Truth, at which Winston Smith works, is tasked with constantly revising or destroying all records of the past which might discredit the Party or Big Brother. For example, Oceania is alternately allied with Eastasia and Eurasia. Whenever the alliances change, all books and newspapers are edited to make it appear that the current ally was always, in fact, an ally, and to eliminate any evidence of shifts in allegiance.
Winston first explains how and why this system works in Part I Chapter 3, saying:
...if all records told the same tale--then the lie passed into history and became truth. 'Who controls the past,' ran the Party slogan, 'controls the future: who controls the present controls the past.' ... 'Reality control', they called it: in Newspeak, 'doublethink'.
It is repeated in Part 3 Chapter 2 by O'Brien, after Winston has been captured and during the process in which he is being tortured and brainwashed into complete submission to the Party.