The Thirteenth Tribe (1976) is a book by Arthur Koestler. It advances the controversial thesis that the modern Jewish population originating from North / East Europe and Russia including their descendants, or Ashkenazim, are not descended from the historical Israelites of antiquity, but from Khazars, a people originating and populating the Caucasus region (historical Khazaria) who converted to Judaism in the 8th century and later voluntarily migrating or were forced to move westwards into current Eastern Europe (Russia, Hungary, Ukraine, Poland, Belarus, Lithuania, Germany and other places outside the Caucasus region) before and during the 12th and 13th century when the Khazar Empire was collapsing.