Khazars

The Khazars were a semi-nomadic Turkic people. They formed an empire called Khazaria in Russia from the 6th to 10th century CE. They came from the Western Turkic Khaganate of the Eurasian steppe, after it was conquered by Tang China.
Khazaria was an international trading center. It was an important place on the Silk Road that linked China, the Middle East, and the Kievan Rus'. For three centuries (c. 650–965) the Khazars conquered the area from the Volga-Don steppes to Crimea and the Caucasus.
Khazaria was between the Byzantine Empire, the steppe nomads, and the Umayyad Caliphate. It helped Byzantine defend itself from the Sasanian Persian empire. The alliance ended around 900. Between 965 and 969, the Kievan Rus replaced Khazaria.
Tengrism may have been the main religion, as it was for the Huns and Turkic peoples. Abrahamic religions were also popular. The ruling class may have converted to Judaism in the 8th century.
Some people think that the Cossacks, Muslim Kumyks, Kazakhs, and some Jews, like the Ashkenazi Jews, descended from the Khazars. Some disagree.
Name.
Khazar or "Xazar" may have come from *Qasar. The Turkic root "qaz-" means "to ramble, to roam" (used in Qazaqsa or Kazakh). It is similar to the Common Turkic "kez-".
Others say it is from "qas-" ("tyrannize, oppress, terrorize") because it is similar to the Uyghur name Qasar.
Others think it is a person or tribe's name. For example the Chinese name Kesa for "Khazars" may be one of the tribal names of the Uyğur Toquz Oğuz (of the Gokturks), namely the "Gésà". But others say Kesa was not a tribal name but the name of the chief of the 思结 "Sijie" tribe of the Toquz Oğuz. Also in Middle Chinese the name "Khazars" always comes before the word "Tūjué" ("Tūjué Kěsà bù": 突厥可薩部; "Tūjué Hésà": 突厥曷薩).
Khazar language is extinct. But modern Turkic languages still refer to the Caspian Sea as the "Khazar Sea".
Language.
No known records of the Khazar language survive. The state was polyglot (spoke multiple languages) and polyethnic. The ruling elite probably spoke an eastern dialect of Shaz Turkic. The ordinary people may have spoken Lir Turkic, such as Oğuric, Bulğaric, Chuvash, and Hunnish. The Persian historian al-Iṣṭakhrī said that the Khazar language was different from any other known language.
After the Khazars became Jewish they may have written in the Hebrew alphabet. Though they spoke a Türkic language, they may have also spoke Hebrew.
History.
Where are they from?
They appear to have came from Mongolia or northern China after the Xiōngnú (Huns) were defeated by Han dynasty of China (Han–Xiongnu war). The tribes probably had Iranian, proto-Mongolic, Uralic, and Palaeo-Siberian people.
The Turkic tribes may have conquered the Western Eurasian steppe as early as 463.
In 552, they conquered the Rouran Khaganate and moved westwards, taking more people from Sogdia.
The ruling family may have came from the Āshǐnà (阿史那) clan of the West Türkic tribes. The Chinese and Arabic records are almost identical, indicating strong support for this theory. The leader may have been Yǐpíshèkuì (乙毗射匱). He died around 651. Moving west, the Khazars reached Akatziroi, one of the important friends of Byzantium fighting Attila's army.
How did it begin?
Khazaria began after 630. It came from the Göktürk Qağanate, after it was crushed by Tang China between 630–650.
Göktürk armies conquered Volga by 549. The Āshǐnà clan whose tribal name was 'Türk' ("the strong one") arrived in 552. They overthrew the Rourans and created the Göktürk Qağanate.
The Chinese Tang Dynasty defeated the Turkic Qağanate and established the Anxi protectorate (Tang dynasty in Inner Asia). The Khaganate split into many tribes. Some tribes went west to the Sea of Azov area. Ashina and the Khazars went further west.
In 657, General Sū Dìngfāng (蘇定方) dominated the Turks and Central Asia. They imposed Chinese overlordship to the east of those Turkic tribes. In 659 the Chinese defeated the remaining tribes. The Khazars did not dare return.
Instead the Khazars defeated the Bulgars further west.
And so the Khazars Qaganate was born, from the ruins of a nomadic empire destroyed by the Tang armies to the east. It became the westernmost successor state of the Gokturks.
The Khazars conquered the lower Volga region and the area between the Danube and the Dniepr. In 670, they also conquered the Onoğur-Bulğar union and made the Onogur-Bulgar language the official language of the empire (lingua franca).
The empire is sometimes called a 'steppe Atlantis' ("stepnaja Atlantida," Степная Атлантида).
Historians often call this period the Pax Khazarica. The state became an international center of trade.
Ibn al-Balḫî wrote in "Fârsnâma" (c. 1100) that the Sasanian Shah (ruler), Ḫusraw 1, Anûsîrvân, said there were three kings who had as much power as he did: the King of China, the King of Byzantium, and the king of the Khazars.