The process of rewriting extremely horrible historical episodes so that those responsible are exonerated is historical revisionism. Simply denying the reality of the event can easily be refuted. The most effective means of historical revisionism is to relativize the evil by pointing out that those responsible are no more evil than anyone else, or the victims of the event are partially responsible.
For many years there have been a host of small groups in the USA, in Canada, in Europe and elsewhere that deny the reality of the Holocaust and endeavour to get this perspective accepted by the mainstream of historical scholarship. Other examples include the genocide of the Herero people in southern Africa, and the collaboration between industrialists of the USA and the Nazi regime. History textbooks in Japan do not refer to Japanese aggression during World War II. Photos claiming to be of Mao's Long March, the storming of the Winter Palace in 1917 and the Viet Minh victory at Dien Bien Phu were taken of staged events, under better conditions. Movie makers are becoming the West's most powerful historians, but not necessarily the most careful. Films like "Mississippi Burning", "Gandhi", "Killing Fields", "Last Emperor" and "Cry Freedom" may be fine or even excellent films, but they are not historically accurate even though they are often thought as such.
In 1990 a committee of historians in the USA accused the government of falsifying the historical record by censoring its publications of official USA diplomatic documents, despite the openness shown by the USSR concerning its own history. The incident concerns the role of US intelligence in post-war foreign policy and the manner in which the published editions of diplomatic archives are sanitized to exclude almost all references to the Central Intelligence Agency, and specifically with the role of the CIA in preparing the coup which brought the Shah of Iran to power and which eliminated the liberal-left Arbenz government in Guatemala. Unmistakeable evidence has been found of dramatic and devastating changes in editorial policies and processes which govern the publication of diplomatic history over past decades.