The Khazars were a Turkic people of the western Eurasian steppe who built a wealthy, multiethnic trading khaganate north of the Caucasus, between the Black and Caspian seas, from roughly the seventh to the tenth century. Sitting at a crossroads of the Byzantine, Islamic, and steppe worlds, they are best remembered because their ruling house and at least part of the elite adopted Judaism, probably in stages, and because they held back Arab expansion into eastern Europe, before the Rus' destroyed their capital around 965.
Sometime in the eighth or ninth century, the king of a Turkic steppe empire did something no other medieval ruler did. Pressed on the west by Christian Byzantium and on the south by the Muslim caliphate, both eager to claim him, he adopted neither. Instead the Khazar court became Jewish, taking up the faith of a people who held no state and no army anywhere on earth. How far the conversion spread, and exactly when, is argued to this day. That it happened at all remains one of the strangest facts in medieval history.