In 838, al-Mu'tasim decided to launch a major punitive expedition against Byzantium, aiming to capture the two major Byzantine cities of central Asia Minor, Ancyra and Amorium. The latter was probably the largest city in Asia Minor 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 "Amorium" 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. Afshin was placed in command of the northern force, that 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 through the Cilician Gates into Cappadocia and head to Ancyra. After the city was taken, the Arab armies would join and march to Amorium. Afshin's force included, according to Skylitzes, the entire Arab army of Armenia, and numbered 20,000 (Haldon) to 30,000 men (Treadgold), among whom were some 10,000 Turkish horse-archers.