The Thirteenth Tribe is a 1976 book by Arthur Koestler advocating the Khazar hypothesis of Ashkenazi ancestry, the thesis that Ashkenazi Jews are not descended from the historical Israelites of antiquity, but from Khazars, a Turkic people. Koestler hypothesized that the Khazars (who may have converted to Judaism in the 8th century) migrated westwards into Eastern Europe in the 12th and 13th centuries when the Khazar Empire was collapsing. Koestler used previous works by Douglas Morton Dunlop, Raphael Patai and Abraham Polak as sources. His stated intent was to make antisemitism disappear by disproving its racial basis. Popular reviews of the book were mixed, academic critiques of its research were generally negative, and Koestler biographers David Cesarani and Michael Scammell panned it. In 2018, the New York Times described the book as "widely discredited." Source: Wikipedia (en)