Ten Lost Tribes
Andrew Gow studied the original German-language texts and concluded that the legend of the Red Jews conflated three separate traditions: the biblical prophetic references to Gog and Magog, the Ten Lost Tribes of Israel, and an episode from the Alexander romance (3rd century AD), in which Alexander the Great encloses a race of heathens behind a great wall in the Caucasus. These traditions had some overlap already; Gog and Magog are among the nations trapped behind the wall in the "Alexander Romance", and the only ones named in the version of the story appearing in Qur'an Sura al-Kahf (The Cave) 18:89, while the 14th-century work "The Travels of Sir John Mandeville" explicitly associates the confined nations with the Ten Lost Tribes.