In 837 Theophilos led a vast army of 70,000 men towards Mesopotamia and captured Melitene and Arsamosata. The Emperor also took and destroyed Zapetra (Zibatra, Sozopetra), which some sources claim as the birthplace of Caliph al-Mu'tasim. Theophilos returned to Constantinople in triumph. Eager for revenge, Al-Mu'tasim assembled a vast army and launched a two-pronged invasion of Anatolia in 838. Theophilos decided to strike one division of the caliph's army before they could combine. On 21 July 838 at the Battle of Anzen in Dazimon, 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.