Battle of Mauropotamos The Battle of Mauropotamos ( ) was fought in 844 , between the armies of the Byzantine Empire and the Abbasid Caliphate , at Mauropotamos ( either in northern Bithynia or in Cappadocia ) . After a failed Byzantine attempt to recover the Emirate of Crete in the previous year , the Abbasids launched a raid into Asia Minor . The Byzantine regent , Theoktistos , headed the army that went to meet the invasion but was heavily defeated , and many of his officers defected to the Arabs . Internal unrest prevented the Abbasids from exploiting their victory , however . A truce and a prisoner exchange were consequently agreed in 845 , followed by a six - year cessation of hostilities , as both powers focused their attention elsewhere . Background . Following the restoration of the veneration of icon s in March 843 , the Byzantine Empire ' s government , headed by the Empress - regent Theodora and the " logothetes " Theoktistos , embarked on a sustained assault on the Byzantines ' main political and ideological foe , the Abbasid Caliphate and its dependencies . This aggressive stance was on the one hand facilitated by the internal stability that the end of the Iconoclasm controversy brought , and on the other encouraged by a desire to vindicate the new policy through military victories against the Muslims . The first such campaign , an attempted reconquest of the Emirate of Crete led by Theoktistos in person , made initial gains , but ultimately ended in disaster . After scoring a victory over the Arabs in Crete , Theoktistos learned of a rumour that Theodora intended to name a new emperor , possibly her brother Bardas . Theoktistos hurried back to Constantinople , where he discovered that the rumour was false , but in his absence , the Byzantine army in Crete was routed by the Arabs . Battle . In 844 , according to Byzantine sources , Theoktistos learned of an Arab invasion of Byzantine Asia Minor , led by a certain ' Amr , probably the semi - autonomous emir of Malatya , Umar al - Aqta . The Arab sources do not make explicit mention of this campaign . The Russian scholar Alexander Vasiliev , however , identified it with an expedition recorded in the poems of Abu Tammam and Buhturi , which was led by general Abu Sa ' id and took place during the regency of Theodora . Umar al - Aqta ' s participation is likely , as he often aided the Abbasids in their raids against the Byzantines . According to Arab accounts , the troops led by Abu Sa ' id comprised men from the border emirates of Qaliqala ( Erzurum ) and Tarsus . The Arab forces united at Ardandun ( possibly the border fort of Rhodandos ) before raiding through the Byzantine themes of Cappadocia , Anatolikon , Boukellarion , and Opsikion . Sa ' id ' s troops sacked Dorylaion and even reached the shore of the Bosporus . Theoktistos led the Byzantine army in against the invaders , but was heavily defeated at Mauropotamos ( " Black River " ) . The location of the latter , if indeed it is a river and not a simple toponym , is disputed ; it was most likely a tributary of the Sangarius in Bithynia or of the Halys in Cappadocia . Not only did the Byzantines suffer heavy casualties , but many senior Byzantine officials defected to the Arabs . Theoktistos returned to Constantinople , where he blamed Bardas for the recent defeats and had him exiled from the capital . Aftermath . The Abbasids were unable to exploit their success due to the internal instability of the Caliphate . Likewise , the Byzantines preferred to focus their strength against the ongoing conquest of Sicily by the Aghlabids . Thus , a Byzantine embassy was sent to Baghdad in 845 , which was warmly received . The Abbasids reciprocated with an embassy to Constantinople , where the two states agreed on a truce and a prisoner exchange , which was held at the river Lamos on 16 September 845 . A winter raid by the Arab governor of Tarsus shortly after failed disastrously , after which the Arab - Byzantine frontier remained quiet for six years .