Khan Tervel or Tarvel, or Terbelis in some Byzantine sources, was the khan of the Bulgars from 700 or 701-718. Not only did he consolidate the Bulgarian settlements in area bordering modern Bulgaria and Romania, but developments of the neighboring Byzantine empire also expanded his domain southwards through favorable treaties and ensured him the upper hand in diplomatic relations between the two states.

Khan Tervel succeeded in either 701 or 702 after his father Asparukh presumably killed in a battle against the Khazars. Shortly afterwards, the deposed emperor Justinian II fled to his place and lured his support. With an army of 15,000 horsemen Justinian suddenly pounced upon Constantinople and slew his rivals Leontius II and Tiberius III, with thousands of their partisans, and once more ascended the throne in 704. Justinian awarded Tervel the marriage of his daughter, title of Caesar which made him second only to the emperor and the first foreigner in Byzantine history to receive such a title, and the best of all, the strategic region of Zagora in eastern Thrace.

Justinian II struggled in the following years attempting to recapture Zagora. Tervel routed him at the Battle of Anchialus or Anchialo in 708 and raided through Thrace to siege the Byzantine capital in 712. In the famous treaty of 716, the Byzantines paid annual tribute to Tervel, furthered their commercial relations and recognized the border in Thrace, where the Bulgars would retain Zagora. Arabic expansion at the expense of the Byzantine Empire presumably precipitated the truce.

Though both states viewed each other as rivals, they joined forces in expelling the Arabs in the Battle of Constantinople in 717-718 . Unsuccessful naval invasions of Constantinople forced the Arabs to land to the west of the capital where the Arab forces led by Maslama, the brother of Caliph Omar II (715-720 AD), was defeated by Bulgar horsemen in the battle.

At his time of the Bulgarian history, the Bulgars was still a nomadic ethnicity within the larger but weaker Slavic population and knew little of the sophisticated knowledges of technology and commerce. Treaties concluded by Tervel accelerated the Bulgar establishments in Balkans although his successors could not follow up his contributions. It would be almost one century later when the ambitious but ruthless Krum restored the Bulgarian glory.

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