The Thirteenth Tribe is a 1976 book by Arthur Koestler, in which the author advances 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.