The Khazars are a semi-nomadic Turkic people originally from Central Asia, who established a khaganate (
empire) in
the Northern Caucasus along the Caspian Sea, in the seventh
century. In it zenith it had controlled most of the Crimea (now the autonomous republic in Ukraine) and the northeast coast of the Black Sea. Their hegemony extended to other nomadic tribes including the Pechenegs, Oghuz, Magyars and the Avars. In the eight century, the powerful
empire converted to Judaism, thus maintaining their neutrality as their kingdom was in the midst of the growing population of the Muslims in the east and Christians in the west. Aside from Judaism, some of the Khazar-controlled areas practice, to some extent, Christianity(Greek Orthodox, Nestorianism), Zoroastrianism, and Islam.
Their first significant appearance is when their ruler Ziebel aided Byzantine emperor Heraclius in the latter's campaign against the Sassanid Persians, overrunning Georgia later. Next, the Khazars fought a series of wars against the Umayyad caliphate in the seventh and eighth centuries with success. Alliance with Byzantium ensued with the marriage of prince Constantine (Constantine V) son of Leo III to princess Tzitzak, later baptized as Irene.
Then once again the Khazars are at odds with the Umayyads. Earlier victories against the Arabs in Mosul ended with their defeat and brief occupation of their capital, Atli. But they regained their independence when the conquerors withdrew due to the instability of the Umayyad regime.In the tenth century the Khazar empire began to weaken. Their alliance with Byzantium collapsed, probably because of their conversion to Judaism. Sporadic raids by various Turkic tribes, Vikings, and Rus' were repulsed successfully, but tables have turned when the Rus' under Svyatoslav I of Kiev destroyed Khazar power with the sacking of Atli in 965, after which they became subjects of of Kievan Rus'. Eventually, after much displacement by the Kipchaks and the Mongols, the Khazars vanished as a nation.