 The Caliph took over preparations for a retaliatory expedition himself , as the campaigns against Byzantium were customarily the only ones where caliphs participated in person . Al @-@ Mu 'tasim assembled a huge force — 80 @,@ 000 men with 30 @,@ 000 servants and camp followers according to Michael the Syrian , or even bigger according to other writers — at Tarsus , and declared his target to be Amorium , the birthplace of the reigning Byzantine dynasty . The Caliph reportedly had the name painted on the shields and banners of his army . The campaign began in June , with a smaller force under Afshin attacking through the Pass of Hadath in the east , while the Caliph with the main army crossed the Cilician Gates on 19 – 21 June . Theophilos , who had been caught unaware by the two @-@ pronged Abbasid attack , tried to confront Afshin 's smaller force first , but suffered a major defeat at the Battle of Dazimon on 22 July , barely escaping with his life . Unable to offer any effective resistance to the Abbasid advance , the emperor returned to Constantinople . A week later , Afshin and the main caliphal army joined forces before Ancyra , which had been left defenceless and was plundered . 
