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.

On the Byzantine side, Theophilos became soon aware of the Caliph's intentions and set out from Constantinople in early June. Theophilos personally led a Byzantine army of 25,000 to 40,000 men against the troops commanded by al-Afshin. Afshin withstood the Byzantine attack, counter-attacked, and won the battle. The Byzantine survivors fell back in disorder and did not interfere in the caliph's continuing campaign. The battle is remarkable for being the first confrontation of the middle Byzantine army with the Turkic nomads from Central Asia, whose descendants, the Seljuq Turks, would emerge as Byzantium's major antagonists from the mid-11th century on.