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Tervel's wars

Tervel, first mentioned in the Byzantine texts in 704 when the deposed emperor Justinian II came to him and asked for his aid, supported Justinian in an attempted restoration to the Byzantine throne in exchange for friendship, gifts and his daughter in marriage. With an army of 15,000 horsemen provided by Tervel, Justinian suddenly advanced on Constantinople and managed to gain entrance into the city in 705. The restored emperor executed his supplanters, the emperors Leontios and Tiberios III, alongside many of their supporters. Justinian rewarded Tervel with many gifts, the title of kaisar (Caesar), which made him second only to the emperor and the first foreign ruler in Byzantine history to receive such a title, and possibly a territorial concession in northeastern Thrace, a region called Zagore. Whether Justinian's daughter Anastasia was married to Tervel as had been arranged is unknown.

A mere three years later Justinian II himself violated this arrangement and apparently commenced military operations to recover the ceded area. Tervel routed him at the Battle of Anchialus (or Ankhialo) in 708. In 711, faced by a serious revolt in Asia Minor, Justinian again sought the aid of Tervel but obtained only lukewarm support manifested in an army of 3,000. Outmaneuvered by the rebel emperor Philippicus, Justinian was captured and executed, while his Bulgar allies were allowed to retire to their country. Tervel took advantage of the disorder in Byzantium to raid Thrace in 712, plundering as far as the vicinity of Constantinople.

According to the chronological information of the Imennik, Tervel died in 715. However, the Byzantine Chronicler Theophanes the Confessor ascribes Tervel a role in an attempt to restore the deposed Emperor Anastasius II in 718 or 719. If Tervel did survive this long, he was the Bulgarian ruler who concluded a new treaty with Emperor Theodosius III in 716 (confirming the annual tribute paid by the Byzantines to Bulgaria and the territorial concessions in Thrace, as well as regulating commercial relations and the treatment of political refugees), and he was also the Bulgarian ruler who helped relieve the second Arab siege of Constantinople in 717 – 718 by land. According to Theophanes, the Bulgars slaughtered some 22,000 Arabs in the battle.