During this period, the Byzantine throne had been undergoing its usual turmoil of rebellions and assassinations. Two rising generals married daughters of the top general, then vaulted over him to become, one after the other, Emperors. The second one, curiously, was of the ethnic minority Atsinganoi, who maybe maybe were originally from Sind, and who probably were the ancestors of at least some Tsigany, that is, Gypsies. But he became the Emperor Michael in 820, crowned by the Patriarch of Constantinople, while Caliph al-Mamun was taking up power in Baghdad.
A rival Emperor Thomas (another general, crowned by the Patriarch of Antioch) had formed an alliance with al-Mamun and besieged Constantinople. Michael formed an alliance with the Khan of the Bulgars, a Turkic tribe that had come west in the 6th century. By now, Bulgaria was a settled, organized state that competed with Khazars and Avars, other early-arriving Turks. By turning the Bulgars from enemies to allies, Michael had the forces to turn the tables on his rival Emperor and defeat him in 823.
But this type of internal strife weakened the Eastern Roman forces, so that they lacked the ships to defend Cyprus from Arabs in 824. In the next years, Arabs expanded across the Mediterranean to Sicily. They besieged the great Greek city of Syracuse in 827.
Emperor Michael’s only son Theophilus became Emperor on his death in 829. He inherited war on two fronts, against the Arab invasions of Sicily and other Mediterranean ports, and against al-Mamun’s armies in Anatolia. Every Abbasid Caliph had made at least feints to take more territory, expanding the border between Syria and what they called Rum—now modern Turkey—by a few towns in each generation. In this border war, it was traditional for the Caliphs and Emperors to lead their armies personally.
Emperor Theophilus lost some battles and won some; he recaptured Tarsus and had a triumphal procession back home in the capital. But the war didn’t continue to go his way, and soon he was suing for peace, offering 100,000 gold dinars to al-Mamun. After al-Mamun died in 833, the truce was negotiated with Caliph al-Mu’tasim (more about him soon).
But Theophilus chose to handle the expiration of the Bulgarian alliance in 836 by invading Bulgaria. The Bulgarians pushed back and reached far into the Byzantine homeland, seizing the cities of Adrianople and Philipopolis (now Plovdiv, still part of Bulgaria). It was a net loss to the Byzantines, who also soon gave Serbia independence.
Theophilus could not sit still for long. He attacked Mesopotamia, seeking to invade the Muslim heartlands. He captured and even destroyed some cities in Armenia. But the Caliph’s army retaliated with an invasion of Anatolia in 838. At the Battle of Anzen, Emperor Theophilus led his own troops against a smaller army led by a Persian prince. Byzantine soldiers panicked when Theophilus apparently went missing (leading a charge, elsewhere). Attacked by Turkish cavalry, they fled. The Emperor and his guard barely managed to escape.
With this victory, the Muslim army could push farther into Anatolia. Caliph Mu’tasim personally captured Ancyra, which became Ankara. His forces also captured and destroyed the city of Amorium, the birthplace of Emperor Michael. It was a prosperous walled city, but they ruined it completely, killing and enslaving all of the citizens. This was one of the greatest defeats the Eastern Roman Empire had suffered yet. With Theophilus’ defeat in his father’s hometown, the politicized doctrine of Iconoclasm collapsed.
Arab forces in the Mediterranean had taken Palermo in 831, setting up an Emirate of Sicily. They hadn’t fully captured Syracuse or other parts of the island, but next they gained a foothold on the mainland. They took the Greek colony of Crotone located on the “sole” of boot-shaped Italy. In 841 Venice, which was allied with Constantinople but mostly independent, sent a fleet of ships to assist the Byzantine attempt to drive out the Arabs, but they failed.
Notice that the Franks, allied to Rome, didn’t seem to take any interest in Sicily and the south of Italy at this point. But they soon did—-next.