Al-Mu’tasim, determined to crush the Greeks, leads…

Al-Mu’tasim, determined to crush the Greeks, leads the largest army yet assembled under one caliph—composed of Arab warriors, Turkish guards, and enslaved men—into Anatolia, where he defeats Theophilus's forces in July 838 at the bloody Battle of Dazimon (now Dazmana, Turkey) on the Halys River.

Al-Mu'tasim thus becomes the first caliph to employ the Turkish mercenaries who will later come to dominate the 'Abbasid dynasty.

In the aftermath of this defeat, and with rumors circulating in Constantinople of his death, Theophilos's position is precarious.

He abandons the campaign and withdraws to Dorylaion, whence he soon departs for the imperial capital.