As the young Theophilos () ascended the Byzantine throne in 829, the
Arab–Byzantine wars had continued on and off for almost two centuries. An ambitious man and a convinced
iconoclast, Theophilos sought to bolster his regime and gain support for his religious policies by military success against the
Abbasid Caliphate, Byzantium's major antagonist. Theophilos launched a series of campaigns against the Caliphate throughout the 830s. These were only moderately successful, but sufficient for the imperial propaganda to portray Theophilos in the traditional Roman manner as a "victorious emperor". In 837 Theophilos personally led a major campaign to the region of the upper
Euphrates, sacking the cities of
Arsamosata and
Sozopetra – which some sources claim as Abbasid caliph
al-Mu'tasim's () own birthplace – and forcing the city of
Melitene to pay tribute and deliver hostages in return for being spared. In response, al-Mu'tasim decided to launch a major punitive expedition against Byzantium, aiming to capture the two major Byzantine cities of central
Anatolia:
Ancyra and
Amorion. The latter was probably the largest city in Anatolia at the time, as well as the birthplace of the reigning
Amorian dynasty and consequently of particular symbolic importance; according to the chronicles, al-Mu'tasim's soldiers painted the word "Amorion" on their shields and banners. A vast army was gathered at
Tarsus (80,000 men according to Treadgold), which was then divided into two main forces. The northern force, under the Iranian vassal prince of
Usrushana Afshin, would invade the
Armeniac theme from the region of Melitene, joining up with the forces of the city's emir,
Omar al-Aqta. The southern, main force, under the caliph himself, would pass the
Cilician Gates into
Cappadocia and head to Ancyra. After the city was taken, the Arab armies would join and march to Amorion. Afshin's force included, according to
John Skylitzes, the entire army of the vassal Armenian princes, and numbered an estimated 20,000 (Haldon) to 30,000 men (Treadgold), among whom were some 10,000 Turkish
horse-archers. On the Byzantine side, Theophilos became soon aware of the Caliph's intentions and set out from Constantinople in early June. His army included the men from the Anatolian and possibly also the European
themes, the elite
tagmata regiments, as well as a contingent of Persian and Kurdish
Khurramites. Under their leader Nasr (converted to Christianity and baptized as
Theophobos), these people had fled religious persecution in the caliphate, deserted to the empire in the previous years, and formed the so-called "Persian
tourma". Setting up camp at
Dorylaion, the Emperor divided his forces: a strong corps was sent to reinforce the garrison of Amorion, while he himself set out with the remainder (circa 25,000 according to Haldon and 40,000 according to Treadgold) to interpose himself between the Cilician Gates and Ancyra. ==Battle==