From the cesspool that is Wikipedia:

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Year Zero is an idea put into practice by Pol Pot in Democratic Kampuchea, that all culture and traditions within a society must be completely destroyed or discarded, and a new revolutionary culture must replace it, starting from scratch. In this sense, all of the history of a nation or a people before Year Zero would be largely deemed irrelevant, because it would ideally be purged and replaced from the ground up.

The first day of "year Zero" was declared by the Khmer Rouge on 17 April 1975 upon their takeover of Cambodia in order to signify a rebirth of Cambodian history. Adopting the term as an analogy to the "Year One" of the French Revolutionary Calendar, Year Zero was effectually an attempt by the Khmer Rouge to erase history and reset Cambodian society to a zeroth year, removing any vestiges of the past.

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Knowledge of anything pre-Year Zero was prohibited. To ensure that there was no recorded memory of a pre-Year Zero society, books were burned. (Wearing glasses was also criminalized as it was taken to indicate that one might habitually read books.) In Democratic Kampuchea, the only acceptable lifestyle was that of peasant agricultural workers. Centuries of Cambodian culture and institutions were thereby eliminated—shutting down factories, hospitals, schools, and universities—along with anyone who expressed interest in their preservation.

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The Khmer Rouge's takeover was rapidly followed by a series of drastic revolutionary de-industrialization policies which resulted in a death toll that vastly exceeded the toll that resulted from the French Reign of Terror.

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